Are you talking about the right side of the frame where the right side safety would go if the gun had ambi safetys.
I had a TM 4.3. There is a small plastic insert that is held on by a very small screw. This part does not show up in the manuals IIRC.
The purpose of this part is to fill the gap on the hi capa frame when the gun comes with a single sided safety. If someone were to say put an ambi safety lever they would have to remove this part. You suppose you could make a new one out of an old credit card if you really needed it to be there.
The part you are referring to would go where the safety goes into the grip right.
