[COPO]

I own the 67 BM Chevelle you mentioned. The POP lists the L-78 and the 4.56 gears (later changed by Motion to 4.88's) The original magazine article states it had an M-20 and the original owner confirmed this, however you are correct the POP only indicates a muncie, not which one, and the original trans is no longer with the car.

Another interesting one is the former Capt. Carl '69 Garnet Red COPO Camaro that received an M-20 from the factory and has original paperwork to prove it.