Originally Posted by Vintage.Singers.NYC

Nice find Annie, as others have noted you've got a model 15 commissioned in 1950 or 1951, with the RAF decals, produced in Singer's Kilbowie plant in Scotland. The machine is interesting/unusual (at least to Americans) for the RAF decal set, uncommon over here; you'll also notice it has the floral-patterned faceplate, whereas a model 15 from the same era produced here in the 'States had the striated faceplate.

Because your machine has the belt-driven motor, it is easy to convert to treadle; you just pop it in a treadle cabinet, add a belt and you're good to go. You can't really do that, at least not easily, with the 15-91 because that has a different motor and handwheel set-up.

As for the sub-model, I do not believe that's a 15-90--I think it's actually a 15-96 or a 15-98, which were originally handcrank or treadle machines to begin with. I suspect your machine was converted to electric and had the motor added to it after the fact. The giveaway is the silver-knobbed stitch length lever. If your machine was a 15-88, 15-89 or 15-90 it would have a round stitch-length indicator plate and a flat silver lever, not the knobbed lever set-up on your machine. Does the motor say "Singer" on it, or is it a different brand?

As far as what it's worth, I'm sure you've heard this before: It's worth whatever you can get someone to pay for it. I've found no matter what machine you're buying or selling, you'll always find out afterwards that someone else paid half that and another person paid double.