[rotorwrench]

I went back and looked at the manual I have for the 1951 thru 53 M-O-M since it is close to what you have. There certainly could be a problem with the manual control valve that is actuated by the gear selector. The small lever has to be able to push and pull the manual valve. They have few good illustrations so a person has to know how that manual valve indexes with the actuating pin in the manual detent lever. There should never be any reverse function anywhere from neutral on down to low gear selection. If there is then I'd suspect that manual control valve function.

It is a busy operation to get that control valve body back in correctly with the two control systems, the park pawl, and the interconnecting tubes all having to go together pretty much simultaneously.

When I compare these old beasts to the modern electric shift transmissions, I can safely say that the turbine rotor, planetary reduction, and shafting is a lot more complicated than the old ones but the old manual hydraulic control systems are way more complicated than the electrical solenoid systems due to their manual design. Modern ones don't even need a governor and all the valves are moved by the electric solenoids tasked to do each job. They are a lot easier to assemble due to that.

A person can likely put a medium case unit in the earlier cars. They are a heavier duty unit but still a lot alike. They all either fit a 223 6 cyl or a y-block until the FE & MEL engines showed up. The Y-block bell housing might even fit the earlier flathead units but I've never tried it.