Major issues with my 1972. Is an alternator from a 1975 internally regulated?

[1972_Buford]

Just bought my 1972 truck for $800 and battery is dead. Battery is brand new. I took the alternator in and it tested bad. I learned from the connectors on the alternator it is not from a 1972 but from a 1975. Were the 1975 alternators internally regulated? Reason I am asking is because the external regulator is still hooked up.

The problem I am having now is, I installed the new alternator (1975) and tested the alternator with the volt meter at the battery. Ended up with 11.8v while running......

I am guessing the alternator is not hooked up properly but would having both internal and external not allow the battery to charge?

Thanks
B