Are the Ultra Compact FSSK and the Classic Style FSSK case swappable? Or is there some difference in the internal components? Also, can you elaborate a bit on the spacebar issue? In the pictures, it seems a little arched but it’s not too bad. Can I fix this issue by replacing the space bar with a new one?
They are not swappable. I have also noted this on each of the 104/SSK product pages.
Which spacebar issue?
All PBT space bars show warpage, even the IBM originals, and warpage is most visible on the largest key. The warpage is due to the uneven shrinkage rate of PBT. A replacement will have the same issue.
Or do you mean the note about the smaller space bars having about 1mm extra space on the right side? You can see this on the example red FSSK case, which was configured for this keyboard with the smaller space bar to allow for the extra 1.25U key to the right of the spacebar.