My 15-91 would not fit into treadle case I have. My answer was to get a 66. Fits perfectly. No adjusting, etc. The treadle case was my great grandmothers and I had it restored after my grandmother had done some 'decorative' finishing to it. No clue what happened to the machine that had been in it. It's probably in my father's attic and he has no clue. Still working on the 66 to get it into usable shape to put in there.