Having just fought this battle-
My 50mm Nikkor has 39mm and this appears to be the standard.
I got a 105mm Componon cheap, but without the board or ring. So, I started the search for these. I read that the other standard was 25mm, and that my lens should be that size so I ordered a board and ring. Nope, didn't fit. Turns out, the lens takes a 32mm (or 32.5mm?) thread. I got a board off of eBay. Looks like a random piece of aluminum, painted black, with an irregular hole gauged into into it. With some filing, I expect to make the lens fit. I have a ring in the mail, and I hope this ends the problem. If that works, I will have spent more on the rings/boards than the lens. I will also have spent way too much time hunting it down.
As noted above--there are some 25mm lenses. I have a board for a 23C and a ring if someone needs it.
Matt