I did check thru the parts book and the transfer plate was a common part used on the standard duty transmissions in that period. I also compared the steel plate to the common steel plate for the std. Interesting that the holes in the modified plate are pretty original in size with the exception of 3 holes.
But when you flip it and compare from the opposite side you find there are many holes and slots never drilled in the Reverse manual plate. Makes sense since all the auto shift valve out and not used.

Still do not know what the plastic check ball does, there are two holes in the plate it controls, where as the original was a slot.