I am trying to verify an original trans for a 69 Z28. It is an OCt 68 build, the #'s off the trans are: P8R05, 18S401666,
Tail shaft cast # 3857584
casting # 3925660
How do you confirm that it is an M-21 (so said) because my reference books say that
"after OCtober 21 1968 the suffix letter was added to aid in which trans. Since this is and OCtober 5 1968 code how do I verify that It is an M-21? And secondly, what significance is the "18S......." number code?

18S = 1968 Corvette as the "S" designates St Louis Plant which is where Corvettes were made.

This is a 1968 Transmission built in October of 67 not 1968, it would be P9R05 if it was Oct of 68. I believe M21's had 2 grooves on the input shaft. Looks like a 68 Corvette tranny.

2 grooves M-20, 1 groove M-21, no groove M-22, but on a M-22, you have to pull the side cover and look at the angle of the gears to be sure.It has a ' 68 date on it, for the model year of the car, a Corvette, but was made in ' 67.That's why it doesn't have an A,B, or C code on it.

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The best way to see what you have is to open the case and count the teeth on the input shaft gear.
M20 will have 24 teeth or less on some models
M21 26 teeth
M22 26 teeth also but will have almost straight cut teeth (about 20 deg.)

What would be the case since the tailshaft housing is correct (1969 muncie). If in case this is out of a 68 corvette St. Louis built....what tranny is it? HAS it been pieced together from a previous rebuild?