Paolo
You need to be careful,
You need to verify that the SEIPLA-75 is really 70mm in diameter. The notes that I have indicate that the SEIPLA is 78mm in diameter. What other lens panels does the D659 take? Looking at the other lens panels might verify 70 or 78mm.
There does appear to be another instance of where the SEIPLA-75 is specified for the old 609 enlarger. But the other lens panel, Seimar, I don't know the diameter of it either.

I have no data on the D659 enlarger.
But if it takes a 70mm lens plate then your Sixtif could be used on it.

The design of the enlarger determines what lens is used on what lens panel.
Example, the 50mm lens on the L1200 uses a flat Lapla lens panel, while on the L1000 it uses the recessed Setopla lens panel.