Not sure if this has ever been posted on Third Gen or not but here's a complete list of every RPO code GM ever produced.

http://aplsweb.com/Topics/TopicPix/RPO.pdf

 

not sure if i should really bump this but it helped me out

 

Absolutely.
Actually it's a pretty good read. Especially engines. (L98 and L46 have been left out and a L88 is a turbo 4-cylinder but THAT'S OK)
The neat thing is that prototypes that never made production and future engines are in there:
LTT = Engine, GAS, 12 CYL, 7.5L, SFI, V12, DOHC, VVT, ALUM, TURBO, GM
(What ya' gotta buy to get one of those? Don't tell me a turbo 460ci aluminum V12 with dual overhead cams ain't got some serious power)
LN1= ELECTRIC MOTOR, 102KW, AC, 312V, TO BE USED FOR ELECTRIC VEHICLE
(lemme guess, batteries not included, right?)
LZ4 = ELECTRONIC SYSTEM, INTERNET CONNECTION
(a car powered by the internet? Is that you Al Gore?)
KPP = FLEET SALES, KRISPY KREME
(now that option has possibilities, I guess it goes with the following...)
B9R = FLEET INCENTIVE, STANDARD COFFEE SERVICE COMPANY
(who says GM can't build car that people will buy. Top THAT Toyota!)

 

I want a V12 powered Krispy Kreme truck!!