I thought a list of the different master cylinder codes could be helpful. The codes appear in the 68 and newer Buick, Pontiac, and Chevrolet chassis service manuals. Anyone able to add the Oldsmobile codes (section 5 of the manual) or the 1970 Chevy?
I kept the list short and simple, so there may be more info in the service manual.
Chevrolet 1968
AT - drum full-size
BS - Drum, manual and power Camaro, Nova, Chevelle
AU - metallic manual Camaro, Nova, Chevelle
DC & PG - Corvette
SA - power disc Chevelle & full-size
AD - disc manual Camaro & disc manual & power Nova
WT - power disc Camaro
Chevrolet 1969
AT - drum full-size
BS - Drum, manual and power Camaro, Nova, Chevelle
DC & PG - Corvette
TA - power disc full-size
BA - power disc Chevelle
US - power disc Camaro Nova
Chevrolet 1970
EB - power disc Camaro
BB - power disc Chevelle
UT - power disc, Nova
Pontiac 1968
HU 1" bore - drum power and manual, full-size
CT 1" - drum power and manual, Firebird & Lemans
FR 1 1/8" - disc power and manual, Firebird
GD 1 1/8" - manual disc, Tempest A-body
DW 1 1/8" - power disc, Tempest A-body
Pontiac 1969
HU 1" - drum power and manual, full-size
CT 1" - drum power and manual, Firebird, Tempest, & Grand Prix,
GA 1 1/8" - power disc, Firebird
EA 1 1/8" - power disc, Tempest & Grand Prix
RA 1 1/8" - power disc, Pontiac (full-size)
Pontiac 1970
EB - power disc Tempest & Grand Prix & Firebird
RA - power disc Pontiac (full-size)
Pontiac 1971
EB - power disc Firebird
Buick 1968 & 1969
CT - drum power and manual
DW - power disc, Skylark A-body
LX - manual drum, LeSabre
HU - power drum, LeSabre Wildcat Riviera
YM - power disc, LeSabre Wildcat Riviera
Buick 1970
CT - drum power and manual
EA - power disc Skylark A-body
LX - manual drum LeSabre
HU - power drum LeSabre Wildcat Riviera
RA - power disc LeSabre Wildcat Riviera
Oldmobile
HU - 67-69 power drum
More info on Camaro brakes:
http://www.camaros.org/suspen.shtml#Brakecomp
5460346 and 5469309 were the oval disc brake master cylinders. The 346 was used 67-68, 309 was 69 and later.
5470409 was the rectangular master that came out in 1970.