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 (69) 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 1968. For example, a production date of 299 would be October 25, 1968 or, the 299th day of the 1968 calendar year. (1968 was a leap year, 366 days). January 1 of 1969 would be 367. A Julian date calendar can be found here to assist you. Remember, a production date of 001 to 366 is calendar year 1968 and 367 or higher is calendar year 1969. If the production date is over 366, subtract 366 and find the corresponding Julian date for 1969. It's not known when the last TH400 was installed in a 1969 Chevelle, but assuming July 31 the last model build day, a date of 578 should be the largest date. July 31, 1969 (578) is the 212th day of 1969.  Subtracting 366 from 578 gives you 212 - July 31, 1969.

Manual, Powerglide, and TH350 Transmission Designations
3-speed 4-speed Powerglide Turbo Hydramatic 350
D - Saginaw **
O - Saginaw **
H - Heavy Duty Muncie
M - Muncie
K - McKinnon Industries
S - Saginaw
W - Warner Gear
N - Muncie
P - Muncie
R - Saginaw
C - Cleveland ***
E - McKinnon Industries
T - Toledo
X - Cleveland (prior to 8/12/68)
B - Cleveland
Y - Toledo
** One source shows the letter 'D' and one source shows the letter 'O'.
Once again the coding for the Muncie 4-speed changed in 1969, but only slightly. The basic code remained the same with the letter designating the month and the model year was included. For example, P9A01A breaks down to P (Muncie), 9 (1969 model year), A (January), 01 (day), M20 . Any Muncie build in calendar year 1968 for an August through December built 1969 model year would be dated with M through T (August through December) such as P9R25A meaning the transmission was coded for a 1969 model year Chevelle on October 25, (calendar year) 1968. A letter suffix code was added in 1969 to identify the M20, M21, and M22 series; "A" for the M20, "B" for the M21, or "C" for the M22.

*** Two Protect-O-Plate cards have been contributed that show C9D03 codes for the transmission.  One a Baltimore L78 4-speed and one a Framingham L78 TH400.
Year - 9      
Calendar Month
M - August P - September R - October  
S - November T - December A - January  
B - February C - March D - April  
E - May H - 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 B9C14 for Cleveland (B), 1969 (9) March 14 (C14).

A Turbo Hydra-Matic 350 would be coded like Y9B26 for Toledo (Y), 1969 (9), February 26 (B26).

Turbo Hydra-Matic 400 3-speed Automatic Transmission
*A sample TH400 code would be 69C223 where "C" indicates a Chevelle or Camaro w/396 engine and 223 indicates August 10, 1968. (1968 was a leap year)
The 3rd character (letter) is designated as follows:
A - 350 cid/255 hp
C - 396 cid/325 hp
D - 350 cid/250 hp
E - 396 cid/350 hp
F - 396 cid/350 hp (Fremont)
Y - 396 cid/375 hp (high shift point)