I’m sharing my experience for anyone who has a C90 and is frustrated because, like me, they can’t find visual back that fits.
However, I just received a visual back that fits my Vintage Orange Celestron C90 (focuser ring type) perfectly!
I’ve been looking for one for some time now so that I can use my quality 1¼” eyepieces and diagonals. Apparently, although there is a 34.6mm (often referred to as 35mm) thread on the back of the C90 the common attachment method is the ~0.965” (slightly larger) opening on the mirror cell back (with knurled thumb screw) which circumvents the use of the threads for attachments. The threads were probably intended for camera attachment with T-Adapter. I purchased a couple 0.965” Antares Plössls for better viewing but wanted more options (BTQ they're very nice eyepieces).
Doing exhaustive searches for a C90 visual back I did find an old CN thread on this subject but it did not provide a viable solution. Some even speculated that such a beast never existed in the first place.
I finally found the EXVB Visual Back for EXT 90/105/125 & C90 Telescope Models online at Telescope Adapters. This works perfectly for my C90 and gives me the solution for which I was searching.
My only “complaint” is that at $38.95 it seems a bit pricey for what it is (small short empty aluminum barrel threaded at one end with a knurled thumb screw at the other end). The upside is that shipping is included in the price and it is a quality hard black anodized item.
My rationalization for the purchase price is that it is roughly the cost of just one decent 0.965” eyepiece and it now allows me to use almost all but my 2-inch eyepieces on my C90.
Had I found this item earlier, I would never have purchased 0.965” eyepieces (gave those up with my boyhood Japanese refractor).
I hope this may help those with a similar want.