I guess this is kind of an old thread, but the best way I know to identify a transmission is by the shape of the case. Chevrolet offered Clark, Spicer and New Process transmissions. Here are some drawings of the cases. I don't think the shift knob is original. In 1965 it was expected that the driver would figure out the shift pattern. It was later that they started marking the shift knobs in the larger trucks.

https://photos.app.goo.gl/RmQzXkKd4owJfv2TA