[Mr70]

Welcome.
I think you've pretty much deciphered it.
Are you sure that's not the letter C before T3?..that would be more reflective of what I have seen.
That 3 may be standing for 1973,as the designated warranty year.
It sounds like it is holding a sequence number.
Muncie did assemble replacement transmissions for Chevrolet starting with sequence #25000,up to #44999 during that particular model year.
That 3925661 maincase was shared by M-20-M-21 & M-22 as well.
1972 was the last year the M-22 was installed in production vehicles also.