Usually painted on top of transmission Th400 or a stamped steel tag attached to the cover...For Muncies. I have not seen a Muncie FF or FX painted on top of case, but have never actually owned a car that I could confirm. After 39 years and a few rebuilds, time has usually removed any traces of painted on two letter codes.

Try this for now..
http://www.chevcarclubvic.org.au/technical/gm_transmission_identification.htm
http://www.drivetrain.com/saginaw3_4sp.html
http://www.gtoalley.com/transns.html
http://www.camaros.org/trans.shtml
http://www.firstgenfirebird.org/firebird/FAQ/trans_clutch/trans.html

The 3 speeds will have the FK in yellow on right side of case, casting 9789553. RJ is part number 3968036.