From Keh.com a "BGN" 90/2 for M from about 1970 (no serial number!) that indeed looks like it has spent much time in a camera bag, but the glass is pretty good, just a couple little cleaning wisps, and all the mechanicals are smooth, for about half the price that 90/2's are going for, even from that vintage. Plus a 50/3.5 from 1954 in really good condition. This is my first collapsible lens, and I'm not so sure if I'm sold on the idea, but it is tiny when collapsed into my M6TTL. I just got some Adox CHS 50 from Freestyle which should let me use both wide open for testing this weekend. The 90 head is removable for use with my visoflex/bellows for macro work, which will be nice.
I don't have a problem, I only have 3 90mm M lenses, and 3 50mm, just 3 each, honest, that's no problem...