The 5660 main case was used for all 4 speeds from 68-70, not just the M22. The M22 would be distinguished by the large ( TH400) output yoke and a drain plug near the bottom of the case.
I've seen many VINs stamped on the top too.
In likethat's pic, the red box is the build date. If you look above that, you can see a VIN stamped in small letters
If the build date of your car is before Feb 12, there's no chance it's original. If after, then I would try to get a small mirror and look on top. Probably be very difficult to see/read if dirty.
1967 GTO; Gulf Turquoise on Black, 4 speed, buckets SOLD
1967 Camaro ss/rs 350
1966 Chevelle SS 396
1966 Chevelle Malibu
1970 Chevelle SS 396
1962 Impala SS
1952 Chev 2 ton flat bed
1938 Chev coupe