The 4.11 ratio is stamped GM so it never came in a Pontiac A body, likely a chevelle set of gears. Chevy did make a 37-9 set for chevelles.
'Ol Pinionhead (member here
https://forums.maxperformanceinc.com...r.php?u=141176)
is the member that will be able to tell you exactly what you're looking at, as well as what it fits. Probably one of the top guys that know every GM rear axle, and parts of that era just from looking at it.