The transmission identification code will differ depending the type of transmission - Manual or Powerglide.

The manual and Powerglide transmissions will have a production code consisting of a prefix letter designating the assembly plant, a production date (month number/day number) and possibly a shift code for day or night shift if the assembly plant was using two shifts.

Manual and Powerglide Transmission Designations
S - Saginaw (3-speed w/overdrive)
M - Muncie
K - McKinnon Industries
H - Muncie (H.D.)
W - Warner Gear
K - McKinnon Industries
R - Saginaw #
P - Muncie
C - Cleveland
*T - Toledo
E - McKinnon Industries
Muncie 4-speed
The first four digits of a Muncie 4-speed are the month/day; example P0310 is March 10.
# There is strong evidence the Saginaw M20 wide ratio transmission did not appear before February/March of 1966, so all M20 4-speeds are assumed to be Muncie before this date.  If anyone as documented proof, such as a warranty card (Protect-O-Plate) showing a Saginaw coded M20 (R) prior to February/March of 1966 is encouraged to send a readable image of the warranty card for verification.
Month Number
08 - August09 - September10 - October
*11 - November12 - December01 - January
02 - February03 - March04 - April
05 - May06 - June07 - July
Day Number
Powerglide Transmission
Normal code would be the plant letter, model year, month/day, shift (Day or Night) such as T1109D for Toledo, November 9, day shift.