This might be Too Much Information for you folks, but some may be interested: the following cribbed from Discogs -can't remember exactly where)
LABELS
Record company codes (the letters after "ST")
A = Atlantic Records
C = Atco
CAP = Capricorn
CTN = Cotillion Records LPs
CO = Cotillion Records 45s
FC = Famous Charisma
SS = Swan Song
AS = Asylum
MC = Manticore
RS = Rolling Stones Records
HT = Hilltak Records
IL = Island Records
Pressing Plant Codes for LPs:
PR = Presswell Records Mfg. Co., Ancora, NJ (they handled most of Atlantic's LP's during much of this period)
LY = Shelley Products, Huntington Station, NY
SP = Specialty Records Corp., Olyphant, PA
MO = Monarch Record Mfg. Co., Los Angeles, CA
PL = Plastic Products, Inc., Memphis, TN
RI = Philips Recording Co., Richmond, IN
AR = Allied Record Co., Inc., Los Angeles, CA
CT or CTH = Columbia Records Pressing Plant, Terre Haute
CP = Columbia (Pitman, NJ)
CS or CSM = Columbia (Santa Maria, CA; very infrequently)
BW = Bestway Plastics, Mountainside NJ
SO = Southern Plastics
MG or MGS = MGM Record Mfg. Division, Bloomfield, NJ
FT = Fidelatone Mfg.
LP label matrix example: (Led Zeppelin I)
ST-A-681461-MO
ST = stereo
A = Atlantic Records label
68 = last two digits of the year Atlantic first received source (tape) and started working on mastering
1461 = tape master number (for side one in this example)
MO = Monarch Records pressing plant
London Records label matrix suffix:
AL - Allentown Record Co.
P - Columbia, Pitman, NJ
TH - Columbia, Terre Haute, IN
SM - Columbia, Santa Maria, CA
SH - Shelley Products, Ltd., Huntington Station, NY
BW - Bestway Products, Inc., Mountainside, NJ
GL - Decca/MCA, Gloversville, NY
K - Decca/MCA, Pinckneyville, IL
PH - PRC (formerly Philips) Recording Corp., Richmond, IN
W - H.V. Waddell, Burbank, CA
Columbia "360 Sound" Label Progression
1962 - 1963 Black "360 SOUND STEREO 360 SOUND" (no arrows)
1963 - 1965 Arrows were added on either side of "360 SOUND"
1965 - 1970 White "360 SOUND STEREO 360 SOUND"
Warner Bros. Label Progression
Mono:
1958-62 - Grey label, black WB Shield, WB in gold print
1962-65 - Same, but in white print
1965-67 - Gold Label
Stereo:
1958-67 - Gold label
1968-70 - Green Label, Warner Bros. - Seven Arts Records. W7 logo under that.
1970-73 - Green Label, Warner Bros. Records. w/ WB Shield
1973-78 - "Burbank" Label
1978-83? - White or beige label w/ Narrow horizontal lines -OR- ghost WB shields in background
1983?+ - White or tan label, no lines, large WB shield