Hey guys, on page 268 in (Chevrolet by the numbers) it states.. October 21st, 1968 a fix letter code was added to the Muncie trans. to aid in identification.
That applies to 69 and later transmissions, and it's actually the letter A (M20), B (M21) or C (M22).
The transmissions with the strange codes are 68's and earlier.
Ed
