I have what I think are two of the same boards. They are threaded female M90x1 at the rear, are made of light alloy.
One came from Italy, the other from France. The dished one had a 135/6.8 Beryl, the flat one an 85/6.8 Beryl. The lenses are in barrel, what Boyer called a re-entrant mount that they used for lenses sold as enlarging lenses. So I believe that my boards are for an enlarger.
I posted links to images of them on the US LF forum years ago, asked whether anyone recognized them. No responses. See
https://1drv.ms/u/s!AggQfcczvHGN9w7B0k02MHLnKM2_?e=glfx1U and
https://1drv.ms/u/s!AggQfcczvHGN9w9H6-BsrfOGh2j-?e=bAns9j