Ford only made two kinds of Posi's back then. they both had a "L" between the ratio number on the rear end tag, Like 3L50. They were Detriot locker and a clutch type unit.
The detriot locker had a "N" in the cast iron case just above the pinion gear. It stood for Nodular Iron.
How to rebuild one?, I would say, put it together, that would be the rebuild. If its a clutch type , no big deal, If its a detroit locker, take it out and sell it and get a clutch type and lots of beer money also.