The Duodecillion, a 40-digit calculator from 1915, and the Macaroni Box prototype from 1921 – both of which I imagine would also be a fantastic synth controllers.
Telephone Museum = lots of synth ideas
Visited the Telephone Museum in Lincoln today – turns out it is way better than you would expect. The switchboard interfaces, featured here, are reminiscent of amazing, antique-futuro synths.