Ixopla, Siriopla, Lapla; and Ixotub, Siriotub, and Latub.
Herewith a picture of the double-bevelled Ixotub - I think the Ixo prefix applies to anything that fits the 606, and probably some other Durst enlargers as well. I believe the double bevel was so that the lens panel could be inserted either way up allowing normal enlargement on the baseboard from 35 mm, or greatly reduced prints when reversed.
I picked up a Latub soon after I bought my 606 as the 606 came with the flattish Ixopla and a 90mm lens for 6 x 6, and of course a 50mm lens for 35 mm film won't focus unless in a Tub. My solution is seen. The Lapla and sirioplas are a few mm narrower, and the single bevel flange is just wide enough to allow cut pieces of matchstick to be superglued to the point where the flange sits under the 2 lugs in the lens panel holder; another (not seen) is glued diametrically opposite for the thumbscrew to tighten (gently!) onto.
I used the enlarger for both 35 mm and 6 x 6 for years; what you see in the picture has stood up to many lens panel changes.
Try it; it certainly saved me because I had no choice if I wanted to print 35 mm negs. And I couldn't see any distortion or uneven illumination on prints often up to 20 x 16.