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Matrix Numbers Explained
When you look in the lead out grooves of an LP you'll see a matrix number; When you look in a recording sessions book
you'll see that recordings were assigned a matrix number;
There is a Year Code:

1955 = F
1956 = G
1957 = H
1958 = J
1959 = K
1960 = L
1961 = M
1962 = N
1963 = P
1964 = R
1965 = S
1966 = T
1967 = U
1968 = W
1969 = X
1970 = Z
1971 = A
1972 = B
1973 = C
1974 = D
1975 = E
1976 = F
1977 = G
1978 = H
1979 = J
1980 = K
1981 = L
1982 = M
1983 = N
1984 = O
1985 = P
1986 = R
1987 = S
1988 = T
1989 = U
1990 = V
1991 = W
1992 = Z
1993 = A
1994 = B
1995 = C
1996 = D
1997 = E
1998 = F
1999 = G
2000 = H
2001 = J
2002 = K
2003 = L

Letter Code:
Size and Speed:
P = Popular
A = Master Tape
A = EP Stereo
A1 = RCA New York
(also "Webster Hall")
I = International
K = 7 inch 45 rpm
B = EP Mono
A2 = RCA Chicago
("Mid-America Rec. Center")
W = Country and Western
L = 7 inch 33 1/3 rpm
M = Mono
A3 = RCA Hollywood
("Music Center of the World")

R = 12 inch 33 1/3 rpm
S = Stereo
A4 = RCA Nashville
A5 = Recorded in the USA
(Not RCA studio)
A6 = Recorded outside of
A in the third position of master tape serial numbers signified RCA; for labels RCA distributed, the code was B
(For example, "Sugar Sugar" by The Archies, recorded in early 1969 at RCA's New York studios and released on the Calendar label which later in '69 changed its name to Kirshner, bore a master serial number of XYB1-3908).

In the case of Elvis, he never recorded at any of RCA's Chicago studios (either 445 Lake Shore Drive pre-1969 or 1 North Wacker Drive from 1969-73), so this is why we see no Elvis sides with A2 in the third and fourth positions of master serial numbers.

RCA's Hollywood studios - located at 6363 Sunset Boulevard after 1963, and 1510 North Vine Street from 1959 until then - assigned master serial numbers to the bulk of his film sides up to 1967.
(For example, while Elvis recorded "Relax" at Radio Recorders on September 22 1962, master serial number PPA3-2726 was assigned by RCA Hollywood some time in early 1963).
Whenever tracks with such codes were released on 45, the third and fourth characters were supplanted by KM for mono 45's and KS for stereo 45's.

Sometimes custon matrix numbers were assigned by RCA when a session was paid for by a film company, like with the January 15-17 1968 double MGM/RCA recording session.
When the songs were purchesed by RCA from the film company a regular matrix number was assigned for the songs.
With the song Goin' Home - matrix number W3KM-6599 was originally assigned by RCA Hollywood (3) for the song but when the tapes were purchased by RCA from MGM a regular matrix number was then assigned to the song - WPA1-1001

A1 was seen on many Elvis sides recorded between mid-to-late 1967 and late 1968, indicating that the master serial numbers with this code were assigned by, as well as mixed and edited at, the New York studio which, before 1969, was located at 155 East 24th Street in Manhattan (and where Elvis hadn't set foot since 1956). Webster Hall, which RCA also used for sessions in New York, was and is on East 11th Street.

In addition, such numbers were usually assigned a few months after Elvis committed them to tape, whether at a film studio or, say, Western Recorders in Hollywood.

The system of matrix numbers has changed several times and when you're looking and comparing matrix numbers you'll see
that RCA doesn't always apply its own rules.

This leads to the next point, based on what has been seen of sessions by other artists (ranging from Perry Como to The Archies), meaning exactly when these master serial numbers were assigned. I have a guesstimate based on such other sessions, thus:

- WPA1-1001 & 1002 / 1022 - 1030, and 1038 & 1039 (Speedway tracks) were apparently assigned in New York in the early months of 1968;
- WPA1-5766 - 5769 (from Live a Little, Love a Little) were assigned around August of 1968;
- WPA1-8023 - 8050 (Elvis comeback special) were assigned around mid-to-late October 1968;
- WPA1-8091 ("Charro") was assigned in November 1968;
- WPA1-8111 - 8125 and 8135 were assigned either November or December 1968; and
- XPA1-3976 - 3978 (around the time of The Trouble With Girls) were assigned within the first few months of 1969.

Some label discographies (i.e. Michel Ruppli's for U.S. Decca Records) have made such indicators in sessions for British artists, mentioning a U.K. session date and then below in parentheses "(master numbers assigned in NYC, [date here])." I presume if a date on any documents pertaining to these sides existed, that'd be the date they were assigned? If any answer on this discrepancy can be answered, it would be most appreciated.

The matrix number coding system changed in 1963 - Before 1963 some codes had a different meaning:
The pre-1962 code listing is as follows:
Second Character - Label Discription
Third Position - Type or Description
RCA Victor
RCA Camden
Slidefilm - Frequency
VIK label (Discontinued in 1958)
Gramophone (Recorded in England)
Groove label release (up to 1957)
Blues, Rhythm & Jazz
HMV (Automatically recorded outside of USA)
Custom (Recorded at RCA studio's)
Promotion & Premium
Custom (Re-recorded from client's source material)
Custom (Master lacquers furnished to RCA)
Red Seal classical release
Slidefilm - Manual
International (Recorded in USA)
Slidefilm - Universal
Country & Western
Foreign (Recorded outside of the USA)
Fourth Position - Size, Speed & Groove
7 inch Stereo 45 rpm – “Super”
16 inch Mono 33 1/3 rpm – Univ. Transcription
10 inch Mono 78 rpm – Standard
7 inch Stereo Compact 33 Single
12 inch Mono 78 rpm – Standard
12 inch Mono 33 1/3 rpm
10 inch Mono 33 1/3 rpm– Standard Transcription
7 inch Mono Compact 33 – Extended Play
12 inch Mono 33 1/3 rpm – Standard Transcription
12 inch Mono 33 1/3 rpm – Univ. Transcription
10 inch Mono 33 1/3 rpm – Universal (Custom)
6½ inch Mono 78 rpm – Standard (Custom)
5 inch 78 rpm – Special
6 inch Mono 78 rpm – Standard (Children)
7 inch Mono 45 rpm – Extended Play
7 inch Mono 33 1/3 rpm – Standard (Custom)
7 inch Mono Compact 33 Single
7 inch Mono 78 rpm – Standard (Spinner)
7 inch Mono 33 1/3 rpm – Standard (Custom)
7 inch Mono 45 rpm
7 inch Mono 33 1/3 rpm – Universal (Custom)
Tape 7½ inch per second – Double Tracked
10 inch Mono 33 1/3 rpm
12 inch Stereo 33 1/3 rpm
16 inch Mono 33 1/3 rpm – Standard Transcription
7 inch Stereo 45 rpm – Extended Play
The 1963 change dovetailed with the first full year of the existence of the label's infamous "Dynagroove" recording process.

Prior to 1962 (the last year RCA used codes which were first put into effect in the calendar year 1955 as signifying their "New Orthophonic High Fidelity"), EP codes were signified by an H in the fourth position on mono 45's and Z in the fourth position on stereo 45's. (R in the third position up to 1962, and in the second position after 1963, indicated a Red Seal classical release.)

As for other second characters pre-1962, 7 indicated a custom recording made at RCA studios and 8 indicated a custom job whose tape was provided to RCA for mastering at any one of their studios.

This was usually followed in the third position by a letter O (erroneously listed by some printing companies that did label copy for RCA as a number 0) which was classified as Phonograph (such as G7OH for a 1956 custom EP, or GO7H if assigned later in the year when more numbers were needed).

The second part of the system consists of the numerical sequence from 0001 to 9999. If they run out of numbers and need to go into the 10,000 series they switched the second and third position in the first part. Thus HO7W-0808 in reality is actually H7OW-10808.
The same applies to the serial numbers for the March of Dimes transcriptions.
If they run out of numbers after 19999 so to speak they would switch the third and the fourth position to begin with 20,000.

Let us look at some examples now:
"Heartbreak Hotel" was assigned matrix G2WB 0209
LP 'It Happened At The Worlds Fair'
G = 1956
Matrix PPRM 2727/28 Mono
2 = RCA Victor Commercial Popular Music
Matrix PPRS 2729/30 Stereo
W = Country & Western
B = 10 inch Mono 78 rpm (20-6420)
The matrix numbers from the LP come after the last recording "Relax" (PPA3-2726)
P = 1963
A3 = RCA Hollywood
P = Popular

When you compare matrix numbers you'll discover a lot:
One example - The LP 'Girls Girls Girls' was assigned the matrix N2 PP 3293/94 (Mono) and N2 PY 3295/96 (Stereo)

The song with the highest matrix number is "We're Coming In Loaded" (N2 PB 3288)

This means matrix numbers 3289-3290-3291-3292 are missing; In the past there were rumours about missing songs from this session.
It could be that these matrix numbers 3289-3290-3291-3292 are given to songs that were not recorded after all, or a second possiblity
is that the matrix numbers are for "Earth Boy (Movie Version)" - "Plantation Rock" - "Girls Girls Girls (End Title)" - "Mama". These 4 songs got a new matrix number when released for the first time (except "Earth Boy" = Unreleased).

Several times a new matrix number was issued to songs with an existing matrix number like the August 1969 recordings when released on 'Collector's Gold'.

As you can see, there are a lot of surprises - I hope this is of some help in deciphering RCA's byzantine master serial number systems.

(Special thanks to Kurt Rokitta, Henk Muller and William Brown)