[Mr70]

That's a Muncie 4spd. wide ratio 2:52:1 as well.
Used originally in:
*1968 Passenger series w/8.15 X 15 tires w/3:55 axle AND when 396cid or 427cid. equipped.
*1968 Corvette w/o Posi or 3:55 axle.

And Recommended replacement in:
*1965-67 Passenger & Corvettes when NOT close ratio equipped.
*1965 Corvette w/o Spcl. Hi-Perf or Fi.

It was soon superceded in 1970 by Muncie #3964507 for same applications.