Muncie four speed trans
Case 3885010
tail 3857584GM
Cover 3884685
Date stamp( top of main case ) P0122
Vin or serial# stamped on side case 6Z124332
Part # ID Tag# 3890534
No rings on input , pass. side spedo, and drain plug .

http://www3.sympatico.ca/atkinsom/history.html

Muncie four speed trans
Case 3885010
tail 3857584GM
Cover 3884685
Date stamp( top of main case ) P0122
Vin or serial# stamped on side case 6Z124332
Part # ID Tag# 3890534
No rings on input , pass. side spedo, and drain plug .
Date stamp - P0122 is Muncie (P), January 22
Partial VIN - 1966, Fremont, sequence number of original vehicle
Tag - 3890534, L78 M20 (after 11/5/65)
No rings on input - probably replaced over the years as 66 & later M20 shown to have two rings.
Drain plug - are you talking drain plug in the bottom of the main case or fill plug in the side of the main case?
http://www.chevellestuff.com/qd/muncie.htm

Thanks guys. This was for somebody else on another site and I figured this was the best resource for the info. I'll pass it along.
Thanks again...