My understanding is that all engines and transmissions started receiving VIN stamps starting in 1968, not just the high-performance units. I believe it was a federal mandate to combat theft.
If the transmission is stamped, it should appear on the top front lip of the trans, where it is very difficult to see when installed in the car. You may have luck spotting it by looking down the back of the engine from above, if you can squeeze an arm down there to wipe away any grime, and with a good flashlight.
Here is an example from my '70 4-speed, but the location and format/layout should be similar ~ could be stamped anywhere along the top lip though, and may be solid and clear like mine, or weak and haphazard.
Good luck, and let us know what you find!