Hello Irvin,
The 1966 M-22 Transmission input clutch shaft has no circlar grooves. The main case has 2 plugs in the passenger side in upper is the fill plug and the lower is the drain plug.
You need to get information of the stamping of assembly date and VIN stamped on the rear flange of the main case.
Hi Drew:
I would use the muncie M22 two groove and dual plug check as a guide only. I got burned real bad years back on so called 66 M22 that was not. The only way to know for sure is to take the side cover off and look at the gear angle.Some replacement over the counter shafts had no grooves only and some m21's had the dual plug. Also at least up here 66 was a year they were not consistant in stamping the cases with any VIN.If he would post the casting numbers of the case and side cover and indicate if the side cover have studs for the levers which it should that would help along with a picture of the gears.
If that is not true CRASH correct me.
I have a real nice 66 trans no groove 10 spline input,small output, reverse switch and dual plugs but it has no M22 gears in it. Unfortunately I paid M22 prices for it but that was many years ago. Not that I am any smarter now
Just trying to help