FD to EOS adapter question...

I have the Bower FD to EOS adapter and it has the removable glass center piece, it screws in or out and has no stopping point. Is it supposed to be like this? does the position of the glass piece make a difference? there is probably a millimeter or two of in and out movement with the threads?

Maybe there is a piece missing. That sounds weird that there is no ridge to stop the glass.