I think it's a 31-15, too. Or maybe a 31-20, which is the same machine only with the M bobbin, rather than the class 15 bobbin.
I recently had a box made for my treadled 31-15, and had a support put on the box for a modern domestic motor.
I wanted it arranged like this so that I can set it in the middle of a work table and have room on both sides for the pipes to support the Flynn frame, which I have never even taken out of the box. I've been so busy that I haven't had time to play with it since I had the box made, but I'll get around to it one of these days.
If it works out, I want to try to figure out how to convert it to the M bobbin. :)
This is a really common industrial machine - all of the replacement parts are easily available online and the Juki hopping foot for high shank machines works just fine on it.