The Kowa process lenses (which in some versions, as Computars also came mounted in shutters) come with little brass spacers and the following info:
"Some times the barrel lens will come with thin spacers. The trick
is this, for center image use, the more spacers between the rear cell and
the shutter the better. For the corners of the image, no spacer is best.
This is why, in a shutter mount, we often see one spacer as the best overall