The transmission identification code will differ depending the type of transmission - Manual, Powerglide 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 was new to the 1967 Chevelle and coding used a different format. This production code consists of a two-digit model year (67) 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 1966. For example, a production date of 298 would be October 25, 1966 or, the 298th day. January 1 of 1967 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 1966 and 366 or higher is calendar year 1967. If the production date is over 365, subtract 365 and find the corresponding Julian date for 1967. It's not known when the last TH400 was installed in a 1967 Chevelle, but assuming July 31 the last model build day, a date of 577 should be the largest date. July 31, 1967 (577) is the 212th day of 1967.  Subtracting 365 from 577 gives you 212 - July 31, 1967.

Manual and Powerglide Transmission Designations
3-speed 4-speed Powerglide Turbo Hydramatic 400
D - Saginaw (3-speed w/overdrive) **
O - Saginaw (3-speed w/overdrive) **
M - Muncie
H ~ Muncie H.D.
K - McKinnon Industries
S - Saginaw **
W - Warner Gear
N - Muncie
*P - Muncie
R - Saginaw
C - Cleveland
E - McKinnon Industries
T - Toledo
X - Cleveland
B - Cleveland
Y - Toledo
** One source shows the letter 'D' and one source shows the letter 'O'. Several Baltimore warranty cards show "S" for Saginaw even with the 3-speed overdrive.
The coding of the Muncie 4-speed manual transmission changed in 1967. A letter now designated the month and the model year was included. For example, P7A01 breaks down to P (Muncie), 7 (1967 model year), A (January), 01 (day). Any Muncie build in calendar year 1966 for an August through December built 1967 model year would be dated with M through T (August through December) such as P7R25 meaning the transmission was coded for a 1967 model year Chevelle and built on October 25, (calendar year) 1966.
Year - 7      
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 Transmission
Variations have been found for Powerglide transmission designations. Normal code would be the plant letter, model year, month/day such as C7C14 for Cleveland (C), 1967 (7) March 14 (C14). However, plant variations have surfaced where the plant letter should be, a number appears such as 87A12 (Kansas POP). Another variation has a letter where the month should be such as C7803 (Baltimore POP).

Turbo Hydra-Matic 400 Transmission
A sample TH400 code would be 67C298 where "C" indicates a Chevelle or Camaro w/396 engine and 298 indicates October 25, 1966.