Hi - chiming in late, but maybe this site will help: http://www.teufert.net/trans/tranny.htm

I think the PB and 65 indicate a 1965 Pontiac 389 V8 based on the tranny code page on the above link? Don't know what the S means. I have a 1967 Firebird with a TH400 and the long code has the 67 and PX so I'm guessing it's the year and code. I could also be totally wrong...