I am unsure how to mount a lens in a Copal 3 shutter on a Sirnar lensboard. Does the spacing ring go in front of the board (between the shutter and lensboard) or in back of the board (between the lensboard and retaining ring)? I think the spacer should go on the back of the board, but I have seen lenses mounted with the spacer on the front, even with large boards, so I am unsure.