Ignatiu5,
An APUGer sent me a 10mm extension tube/ring that has M39 m/f threads and the male threads go into the lens board and it has female threads available for any M39 lens. It accomplishes the same thing as Ronald described except it can be easily removed by unscrewing the adapter.
After the fact, I discovered some jam nuts I have that have a skirt and three holes drilled into the skirt. I could also have drilled/tapped three holes on the lens board and permanently mounted the jam nut on the outside and had threads available but, again, the original board is now modified.
There are a number of solutions to 'skin the cat' but I'll stick with the jam nut mounted adapter for now.
PS. I have no problems with my 'N' series 50/80 El-Nikkors on the board.