The transmission identification code will differ depending the type of transmission - Manual, Powerglide, Turbo Hydra-Matic 350 or Turbo Hydra-Matic 400. 

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

The Turbo Hydra-Matic 400 used a different date coding format. This production code consists of a single-digit model year (2) followed by an engine/model code letter and a three-digit production date. The three-digit production date begins with the first day of January 1971. For example, a production date of 298 would be October 25, 1971 or, the 298th day of the 1971 calendar year. January 1 of 1972 would be 366. A Julian date calendar can be found here to assist you. Remember, a production date of 001 to 365 is calendar year 1971 and 366 or higher is calendar year 1972. If the production date is over 365, subtract 365 and find the corresponding Julian date for 1972. It's not known when the last TH400 was installed in a 1972 Chevelle, but assuming July 31 the last model build day, a date of 577 should be the largest date. July 31, 1972 (577) is the 212th day of 1971.  Subtracting 365 from 577 gives you 212 - July 30, 1972 (1972 was a leap year).

Manual and Powerglide Transmission Designations
3-speed4-speedPowerglide / TH350
D - Saginaw **
O - Saginaw **
M - Muncie
K - McKinnon Industries
S - Saginaw
W - Warner Gear
N - Muncie
P - Muncie
R - Saginaw
C - Cleveland
E - McKinnon Industries

*B - Cleveland (TH350)
Y - Toledo (TH350)
J - GM of Canada (TH350)
* A typical Muncie will break down as follows: P2A01C breaks down to P (Muncie), 2 (1972 model year), A (January), 01 (day), C (M22). Any Muncie build in calendar year 1971 for an August through December built 1972 model year would be dated with M through T (August through December) such as P2R25A meaning the transmission was coded for a 1972 model year Chevelle on October 25, (calendar year) 1971. The letter suffix code identifies the M20, M21, and M22; "A" for the M20, "B" for the M21, or "C" for the M22.
** One source shows the letter 'D' and one source shows the letter 'O'.
Year - 2  
Calendar Month
*M - AugustP - SeptemberR - October
S - NovemberT - DecemberA - January
B - FebruaryC - MarchD - April
E - MayH - JuneK - July
Day Number
Powerglide 2-speed and Turbo Hydra-Matic 3-speed Automatic Transmission
B - Cleveland (TH350)
C - Cleveland (PG) only available with the base 6-cyl and V8 engines.
Y - Toledo (TH350)
J - GM of Canada (TH350)
Variations have been found for Powerglide and TH350 transmission designations. Normal code would be the plant letter, model year, month/day such as B2C14 for Cleveland (B), 1972 (2) March 14 (C14).

A Turbo Hydra-Matic 350 would be coded like Y2E01 for Toledo (Y), 1972 (2), May 1 (E01).

Turbo Hydra-Matic 400 Transmission Designations
A sample TH400 code would be 72D275 where 72 is model year 1972, D designates engine code/402~240hp & F designates engine code/454~270hp, and 275 is the 275th day of calendar year 1971, or October 2. At least one variation has been posted here where the '72' model year is displayed simply as '2'.

It's possible that when the TH400 transmission was built in calendar year 1972 and the day count was less than 3-digits (i.e..,=<99) the POP would be coded as 72, then the engine letter, then the 2-digit day count such as a known example of 72F53, or February 23 (the 53rd day) which would coincide with the March built date of the car.

Should note here too that in 1972 plants didn't seem to have a standing SOP for transmission coding, be it manual or automatic. If yours make no sense by the rules above, please let me know exactly what transmission you know is correct, the car's assembly plant, and just EXACTLY how the transmission is coded.  Better yet, email me a readable photo to and I'll look at it and get back to you.