Yes, the idea was to squeeze as much as possible out of the negative.
The Universal-Mask with 24.25x18.6mmm would seem to be the best alternative to the ANSI (24.9x18.7mm) as they're almost identical in dimensions. Thank you for the pointers, Robin!
I suppose the DIN 1.33 Mask you have would get me 24x18mm, which in the end I may be able to live with. Let's see.
Do you by any chance have a matching ground-glass in storage as well?