The Opmus was a impulse buy, my main enlarger is a Omega DIII, I have an Omega 6X7 which I had a hard time keeping in aliment, the Opmus is very well made, in perfect aliment. I have Schneider 50MM 2.8 and 60mm 5.6 -both in 39mm mount along a Schneider 80mm 4.5 and a set of Wollesnake Raptiar (sp?) 50mm, 75, 80 in 25mm, the standard Opmus lens board is 23.5. I can have the current lens board drilled to 25mm and with the lens board on the way I could use any of my current lens. I usually have 2 enlargers set up as I work with 2 negatives at a time, usually a 4X5 on the DIII and a 35mm or 6X6 on the other enlarger, so as test prints are washing and drying and move to the next negative .
I think I know what happened to the Belar. The Ompus must have be storage for years, maybe decades, although the box for the Belar was included the lens was left on the enlarger when put back into the storage box. The storage box complete with molded Styrofoam cut outs for each of the parts. The lens coating on the Belar interacted with the off gasses of the Styrofoam and left a film on the lens. The condensers, glass negative carrier or heat absorbing glass insert are clear so it must have the coating. I got most of it with lens cleaner, but still not clear, might have to remove the coating, not work the effort.