The team at Brother way back then must have been quite something. Like any product manufacturing company on the rise they had a mission and by the 60s had become a premier maker of machines.
Another outstanding 15-91 machine could sum it up because that is all it is. Manufacturing guys really like making what they made yesterday and so the incremental changes we see in the Japanese imports are minuscule and usually very well executed.
I'll say here that "all metal parts off" restore is NOT for the newbie, you will end up with a pile of parts that a machine you can't use. If you have no experience with machine repair then I suggest you start out very slow and do not willy-nilly disassemble anything.
The reason that I do "all metal parts off" is because I really get annoyed at the little edge of oil grime between the metal
parts and the painted casting; this is so difficult to get at that the
only way is to remove parts, though you don't have to. Removal also makes the cleanup of those parts easy, the trade-off is the reassembly know-how and lots of time.