Arca Swiss Ground Glass Back, help needed...

hello

I just bought a 2nd hand Arca Swiss 69, it comes with an original ground glass back adapter, but the ground glass itself it a DIY job. It was adapted to carry a mamiya 67 waist lever viewer with magnifier, clever but that's that what I want.

I can see that the Arca Swiss ground glass adapter is made to hold a fresnal as well, on the inner groove, sits a mamiya 67 ground glass, propped up by about 2 mm. On the outer groove, sits a normal piece of clear glass pressing the mamiya 67 ground glass onto the holder.

How is it supposed to be originally ?

I have a nagging feeling the focus is completely whacked.

just to have it clear, the holder is original only the 2 pieces of glass is a DIY job.

cheers