[Mr70]

Everything Chris mentioned is right,plus the fact that the 1970 L-78 equipped Chevelle was not eligible to receive an M-22 transmission on the assembly line from any one of the 5 vehicle assembly plants @ that that time...only an M-20 or M-21 was possible,when ordering from the M/T family.
Your over the counter M-22 looks like it shows it was the 42862nd unit assembled on August 29th,1970 out of the 1000 over the counter service units available from their Muncie plant that year.
You also show it has the #7584 extension housing,which I believe means it was intended to be installed as a service replacement in a Corvette or Camaro..not Chevelle,which used the #8764 housing.
I'm curious to know if your M-22 has fine spline input/output shafts & if it has a drain plug?..as it might have been assembled in August 1971.