Whilst trying to find an answer to the problem in hand I found this site selling Durst lens boards.
It's a handy source to bookmark.
Lensboards for Durst enlargers
Lensboards with 78mm diameter for most Durst enlargers. Available with all common thread sizes up to 72mm x 1. Lensboards can be made with other lens thread sizes.customphototools.com
I have never used them, so cannot comment on the quality etc.
Also no connection.
However it's only a repro of a Lapla 39 which the OP already has and still won't enable the 50mm f3.5 Ronar or Trinar to focus for larger prints.
A good 50mm enlarger lens can be found for around the same price.
Ian
