Bryon, do the gears look M22 to you? Looks like a 73 non M22 unit I have inside the 69 case.
Sorry - I was not following the responses.
My other post was just verifying the stampings looked OK.
As stated, it’s an M21 with one ring on the input shaft. Also the case has a B code after the assembly date, also indicating M21.
I don’t know why the ad says M20.