I found this info here in Apug for Durst D659:
"A Durst SEIPLA-75 recessed lens mount tub is 70mm in diameter and 33.5mm deep and could be easily modded to hold a 50mm lens in place."

I found on a Durst general part list booklet (I have been given together with other things) that the Seipla 75 was good for 75 or 80 mm lenses with 25mm screw mount on enlarger D659.

So my Sixtif could be mounted on a D659 since it has 70mm diameter? And used with longer lenses than those good with M600?