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...Where are you finding M39 to Contax adapters? I'm trying to find one for a Rodenstock 90 and the closest I've come is a funky looking M42 to Contax and a step down ring.
I'm simply suggesting that there are sometimes inexpensive methods that work well. If it's difficult to get an M39 to Contax adapter, but an M42>Contax is easy then why wouldn't you use an M42>Contax even if another adapter/ring is needed in between? I regularly do that with m42 adapters.
By the way, you can also use T2(M42x0.75)>Contax adapter and then use an M39>T2 adapter. I don't know if that's any cheaper, but some enlarging lenses are threaded with M42x0.75, eg. Schneider Componon-s 135/5.6.
In reality, there are very few off-the-shelf adapters other than the usual M42/M39 for most mounts so when you want to use an M32.5, M25, M40x0.75 etc. then you usually have to use multiple adapters anyway.
If you are interested in following it up then PM me for the details of a company I've used for custom adapters. I can't speak for them but they might be able to make an M39>Contax, if you really want to do that.