When I got a 150mm f4.5 Xenar lens it was mounted on a board.
It measures 71mm x 71mm square.
On the back its re-inforced by another plate 62 mm across from which comes a tab 5mm deep with a tiny hole bored through it.

The lens was for a Linhof of some kind as the Synchro Compur shutter is engraved LINHOF.
If you know what camera the board is for it has a #1 hole & you are welcome to it ,cos it's no good to me.