The cc3 could indicate a replacement case.
The drain plug isn't a sure clue either.
Sounds like the gears are the only way you'll know on this one. With the M-22 gears, you can draw a line between the gears without hitting each other...if you can picture what I mean.
What is the casting number?
Is there a number on the mid plate starting with a "P"
All "661" cases have the drain plug, and even some of the "660" cases. I have a "660" M-21 with a drain plug.
Corse spline, or fine spline input?
Lack of "rings" on the input doesn't necessarily mean anything either... sounds like a worked over box with a replacement case..
What did you pay for it??