I think my description of the phenomenon as "concentric rings" may have led some folks astray. Instead of rings, think of them as arcs appearing in the upper right and left corners of an 8 x 10 print. If they were to form circles, the diameter of the outside ones would probably be around 5 or 6 inches. They are so uniform and perfectly formed that one would tend to ascribe them to some sort of optical/mechanical reflection generated perhaps by the enlarger lens, a 135mm Schneider Componar operating at f11.
In the past 18 months or so I have made probably 5 or 6 hundred b&w prints with this enlarger and light source, mostly from 6x6 Rolleiflex negs on Ilford RC paper, and never seen these rings. In the more recent past I have made perhaps a few dozen Ilford RC prints from 4x5 negs with no problems, or at least not [I]this[I] problem.
Very puzzling.