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 two-digit model year (71) 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 1970. For example, a production date of 298 would be October 25, 1970 or, the 298th day of the 1970 calendar year. January 1 of 1971 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 1970 and 366 or higher is calendar year 1971. If the production date is over 365, subtract 365 and find the corresponding Julian date for 1971. It's not known when the last TH400 was installed in a 1971 Chevelle, but assuming July 31 the last model build day, a date of 577 should be the largest date. July 31, 1971 (577) is the 212th day of 1971.  Subtracting 365 from 577 gives you 212 - July 31, 1971.

Manual and Powerglide Transmission Designations
3-speed 4-speed Powerglide Turbo Hydramatic 350
K - McKinnon Industries
S - Muncie
N - Muncie
P - Muncie (alum case)
R - Muncie (iron case)
C - Cleveland
E - McKinnon Industries
B - Cleveland
Y - Toledo
J - GM of Canada
A typical Muncie will break down as follows: P1A01B breaks down to P (Muncie), 1 (1971 model year), A (January), 01 (day), B (M21). Any Muncie build in calendar year 1970 for an August through December built 1971 model year would be dated with M through T (August through December) such as P1R25A meaning the transmission was coded for a 1971 model year Chevelle on October 25, (calendar year) 1970. The letter suffix code identifies the M20, M21, and M22; "A" for the M20, "B" for the M21, or "C" for the M22.
Year - 1      
Calendar Month
M - August P - September R - October  
S - November T - December A - January  
B - February C - March D - April  
E - May G - June K - July  
Day Number
Powerglide 2-speed and Turbo Hydra-Matic 3-speed Automatic Transmission
B - Cleveland (TH350)
C - Cleveland (PG)
Y - Toledo (TH350)
Variations have been found for Powerglide and TH350 transmission designations. Normal code would be the plant letter, model year, month/day such as B1C14 for Cleveland (B), 1971 (1) March 14 (C14). Arlington POPs have shown the letters "HW" following the transmission date code; to date it is unknown what they are for.

A Turbo Hydra-Matic 350 would be coded like Y1E01 for Toledo (Y), 1971 (1), May 1 (E01).

Turbo Hydra-Matic 400 Transmission - this will vary from plant to plant
* One sample TH400 code might be 71183CF where "71" is the model year, "183" is the 183rd day of 1970 or July 1, and "CF" is the code for the 454 engine.
* One sample TH400 code might be 71D43 where "71" is the model year, "D" is the engine code for a 402, and "43" being the 43rd calendar day of the year.