I haven't been here for awhile, but a discussion on FB brought me back to decode my PoP again on a different topic.
My PoP has a transmission code of P9R31, which is a muncie 4speed October 31 without the suffix. This is an incredibly small detail, but the comment in the decode says suffix began the week earlier.
1969 Muncie 4-speed only:
A = M20
B = M21 Use of the Muncie type suffix
C = M22 began with 21 Oct 68 production.