Darren,
If you change the shuttle driver the standard 31-15 parts will fit,the differnce is it's larger but thinner so I doubt if it even holds anymore thread but alot of times Singer would make these certain sub-classes like this to sew certain things a certain way.The bobbincase costs more than the 31-15 one.
Here's one example,a Singer 98-3 made to sew (re-cap) tires,made in 1915 & before I owned it I never heard of it.But in 1915 they didn't make good tires & Singer says the result" is a tire equal to or better than the original & you can also reuse one w/a small blow out that have been vulcanized"
But back then there was alot of different things that were made,that required sewing that we don't use today or are made a different way or different material that made the machine obsolete.Some of them can be adapted to other uses.