I too have an M605K that I use with a Nikon f2.8 50 mm N lens and have never had any problems achieving focus even using the extension arm to get larger images. I am using the recessed lens board. I have never tried the under lens filters with this enlarger so cannot comment on their use.
If you have the under lens filter holder held in place by the lens I wonder if the thickness of the holder is throwing the focus off? Can you focus with the Nikon and Schneider lenses fitted without the under lens filter holder?
Bill
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