Guide to Muncie M20, M21, and M22 Four-Speed …
The early production Muncie four-speed case is the small-bore unit and can be identified by the GM casting number 3831704 (see Muncie M20/M21/M22 Main Case Identification chart). The large-bore Muncie came into production in 1964-'65 and became the standard from then on through the end of production in 1974.