You need a locking ring that treads to the bake of the lens like on large
format. I believe it should be a 39mm ring.

Robert

It may not have been inteneded to be attached directly to the lensboard at all.
Many enlargers used lensboards with unthreaded holes in them.
The lens was held on by a retaining ring which screwed onto it.
Look near the back of the lens, the ring may still be there.

This was done so that you could orient the lens exactly the way you wanted
it. Since there was no standard for length of the threads, when you
screwed the lens into a threaded lensboard, it could end up with the
apeture (fstop) scale facing the back. If you unscrewed it enough to
have it face front, the lens would be loose and could shift focus
as you moved it.

Geoff.