Sounds like you have the anti-newton ring "filter" without the glass. I have never seen a spacer ring to go between the filter and condenser. My anti-newton "filters" are like slip on filters, friction fit to the condenser. All the 1C enlargers are set that way and no such ring is mentioned in the instruction book.

There is also a slip on solid brass "filter" with a negative size cut out that can act as the top half of a sandwich type negative carrier. You lose flatness with it but never get newton rings. I have only seen pictures.

Remove your ring so the condenser sits flat. If you get a print with rings, reinstall it for that print alone.

Depending on the lens you have, you may benefit from a not so quite flat negative. The first lens is optimized for 5x7 prints has pretty bad field curvature and the image falls apart > 8x10. The second with a large front element is dead flat and good to 16x20 wide open. The Focotar II is back to a not so flat field again.. The middle one was a Schneider lens rebranded and is very very good.