I have the enlargers (Omega D5 and Durst L1000) and lensboards (as long as the lens has a 39mm mounting thread), I even have extra 39mm retaining rings.
I know what you mean, I had some lens w/o retaining rings and w/o the retaining rings I could not mount them. So I picked up a few, for my lenses and to keep a spare on hand.
The only stumbling is if I run into a Schneider Componon-S 135mm with a 42mm mounting thread. I don't have a lensboard for that lens.