Back then you could buy a factory built third-member over the counter.

I have an OEM 3.42 geared, 742 case with Detroit Locker. It has a '64 date code and the case is even stamped "3.42 NS" (No Spin) in the correct location and font for the year.

The standard nomenclature for a third-member equipped with a traction differential in the early '60s would be something like "3.91 S" (Sure-grip).