what trans code is this

my wife bought me a72 dart swinger while i was deloyed and the guy she bought it from gave us an extra trans. right now the car has a slant six in it i looked at the casting stamp on the driver side of the trans and i cant find this number anywhere. the stamp is PK3681 after these numbers is looks likeits been stamped again, cant tell if it was first a 821 or 441. any help would be great thanks. did the 727 come behind a slant six at any point, this is my first mopar build been a chevy guy prior to this buy.

thanks