I have an older El-Nikkor 150mm lens with a 53mm mounting thread. I am currently using a darkroom with all Omega D and DII enlargers, all equipped with the Chromega heads. Most lenses mount on the oval quick-change plates with 39mm holes and retaining rings. I have looked on eBay, etc. for a plate with the requisite 53mm opening to no avail.
Pardon my ignorance of Omega equipment (I'm a Beseler guy...). I'm hoping someone with more familiarity with Omega products can point me in the right direction.
I used to have a D2 before people started giving away D5's. The D2 uses round lens plates and lens cones, replaced by bellows in subsequent models.
I had a look in my parts drawer and see that I have two left-over Omega D2 lens plates (one in the standard 39mm mount, the other one looks to be cut to 50 or 51mm, which might fit your lens, or could be filed out further). If you want them, send me a PM and they're yours.