Where the rings came into their own is in colour slide mounting (remember that?) where fortunes were made and lost in the production of special glass mounts to avoid the rings becoming obvious in projection where they move about with film flexing under heat. Not only were the rings seen clearly with plain glass but the surface of the anti newton glass could be focussed on the screen.
Just a few of the avoiding measures taken in the 1950's - some under licence from Leitz, made by PerroColor in Switzerland marked NEWLO for etched glass mounts.Those who projected Kodak slides as they arrived in card mounts had to put up with (unsharp) popping of the film until Leitz came along with curved field lenses late in the day.Note the reliance on masks to maintain separation of film and glass.
Avoidance of the optical effects in prints led to a number of DIY solutions. However, a high tech correction is available. Leica marketed the solution for the Focomat which is an etched plate of Anti-Newton glass in a mount to fit over the condenser with a brass ring to raise the condenser a little - about 3mm, to preserve free movement. The Valoy 11 has a different condenser mounting and the solution was to make the later Valoy condensers available finished in an Anti-Newton surface. You still need the ring to modify an early Valoy, without the later type condenser being to hand, making use of a separate anti-newton glass.
These components have been hard to find at this late stage and command high prices. However some perfectly acceptable Japanese copies have been hand made in limited numbers and one is included in the illustrations below.
I believe the correct codes are NESOO for the Glass disc in mount to fit over the original condenser with the spacer ring required and 17507 for the later etched condenser for the Valoy 11, or 17634 for the Valoy 1. (They certainly think of everything!) Neither have any engraving that will assist identification. An older metal mask offered as a solution was in effect a factory made equivalent of the acetate mask and is listed as NEWOO in some books of reference and dated as introduced in 1951. This photo shows the factory glass plate and adjusting ring, together with the Japanese version which is only finished in black in the light path. The second Brass ring-more a shim- seems to be in place on all Focomat enlargers, I suspect to reduce friction in the condenser lowering mechanism. All comments welcome.
The Japanese product is pleasing and has been in use. You may wish to see the instructions........