the leica manual has a pattern for a shim you can cut out of sheet film that goes over the negative and raises the condenser just a smidgen and prevents newton rings.

or, you can guy an anti-newton ring filter, call E. Leitz in New Jersey, they may still have one in stock. I bought a new power cord for a IC a while back.

But the self-made shim works well -- you cut out a piece of film big enough to cover the negative, punch a couple of holes for those pins and a piece the size of the negative. As I said, if you can locate a Leica Manual from the 50s or thereabouts you can find the pattern, but a moment's pondering of your rig will show you the way.