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 introduced in the 1967 Chevelle and coding used a different format. This production code consists of a two-digit model year (68) 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 1967. For example, a production date of 298 would be October 25, 1967 or, the 298th day. January 1 of 1968 would be number 366. A Julian date calendar can be found here to assist you. Remember, a production date of 001 to 365 is calendar year 1967 and 366 or higher is calendar year 1968. If the production date is over 365, subtract 365 and find the corresponding Julian date for 1968. It's not known when the last TH400 was installed in a 1968 Chevelle, but assuming July 31 the last model build day, a date of 577 should be the largest date. July 31, 1968 (577) is the 212th day of 1968 (a leap year).  Subtracting 365 from 577 gives you 212 - July 31, 1968.

A typical Muncie 4-speed would be dated such as P8K01 and will decode as P (Muncie 4-speed), 8 (model year), K (month - July), 01 (calendar day 01).

Manual, Powerglide, and TH350 Transmission Designations
3-speed4-speedPowerglideTurbo Hydramatic **
M - Muncie
H - Heavy Duty Muncie
K - McKinnon Industries
S - Saginaw *
O - Saginaw O.D.
W - Warner Gear
N - Muncie
*P - Muncie
R - Saginaw
C - Cleveland
E - McKinnon Industries
T - Toledo
Y - Toledo
The coding of the Muncie 4-speed manual transmission continued the same as 1967. A letter designated the month and the model year was included. For example, P8A01 breaks down to P (Muncie), 8 (1968 model year), A (January), 01 (day). Any Muncie build in calendar year 1967 for an August through December built 1968 model year would be dated with M through T (August through December) such as P8R25 meaning the transmission was coded for a 1968 model year Chevelle on October 25, (calendar year) 1967.
* Some plants will code the Saginaw O.D. and standard Saginaw with the letter S and not with O when there is an overdrive transmission.
** There are several examples of the 'new' TH350 transmissions being fitting into 1968 Chevelles for testing. Ones found are Toledo-built with a "Y" designation and the number 9 for the model year. Whether the Cleveland plant built any is uncertain.
Year - 8   
Calendar Month
M - AugustP - SeptemberR - October 
S - NovemberT - DecemberA - January 
B - FebruaryC - MarchD - April 
E - MayH - 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 C8H21 for Cleveland (C), 1968 (8) June 21(H21) or C8C15 for Cleveland, 1968, March 15. However, plant variations have surfaced, particularly Fremont, where nothing seems to make much sense. One Fremont plate has TP08P07 while another has T8P01D. The first probably decodes to Toledo Powerglide (TP), 1968 (08), September 07 (P07) where the second is probably Toledo (T), 1968 (08), September 1 (P01), day shift (D).
Turbo Hydra-Matic Transmission
A sample TH400 code would be 68C298 where "C" indicates a Chevelle or Camaro w/396 engine and 298 indicates October 25, 1967.
The 3rd character (letter) is designated as follows:
A - 396 cid
C - 396 cid
D - 327 cid
E - 396 cid
H - 396 cid