I have been reading up on my recently received 600mm and 750mm apo-germinars (carl zeiss jena model). I believe they are missing the brass shims, the "trimmring" spacers that Arne Croell wrote about. There is a slot in the middle of these barrel lenses that you can open by twisting the lens. I am guessing this is where the shim should go. Is anyone aware of where these things could be bought? I am under the impression that they improve focus at infinity.