Early swing has short axle/spline, later swing has long axles/splines. The "snout" on the drums is longer on lates so you would have to mill some off (I think its about 15mm) to use it on an early car. Mine actually has a later pan under it, so I'm good to use the later stuff. (later is mid 68-on)
Lanner, if I'm completely wrong, don't hesitate to bust me out.
I tried to read up on this stuff and could have it all bass-ackwards.
Jeff-