DR,
Just looked at one of my 31-15s and it has an extended part that the needle positioner would attach too back by the handwheel. I would take a measurement of the end of the handwheel and then double check with the company that it would fit. My two machines did not have that extended piece with the handwheel leaving room for it to attach to easily or at all as some can not have the needle positioner to my knowledge like the 107 Singer I had but the Servo motor was still worth it without the positioner for me. Mine had indentation so I needed a piece that would fit into the handwheel end and into the needle positioner. My cousin used a piece of steel and put them on a lathe to fit both openings. Check your machine but from the one I looked at here I believe you would be able to just slip it over and attach. Ask for both the diameter and depth. I would use calipers if you have them otherwise they are cheap from Harbor Freight and you would use them for other things to.
Hope this helps with your decision or at least makes it a little easier. It is worth the time as I really like mine but lucked out because my cousin is so handy, and that's putting it mildly. He has helped me with lots of stuff and also the reason I have continued with getting the other industrials and I wouldn't trade them for anything. I have Singer 111W155 walking foot with positioner, Necchi straight zig zag 720 100 with needle positioner, Rimaldi Serger 327 with just a servo motor no needle positioner, Singer 176-22 fur machine no Servo yet, Singer 29-4 and a Singer 246 from my Uncle Arnie that started the whole thing!!! I must admit the 246-13 is the best machine but I wanted 4 thread and if I find a turtle style Singer four thread cheap I will exchange the Rimaldi in a heartbeat as parts are so much easier plus it is a machine that so far has never failed or given me much trouble.
Good Luck,
Lisa