Hi! Welcome!
It is an RAF decal, and a 15 model, likely a 15-90 - or at least that's what it is in its current incarnation. It may have started out life as another model 15 but likely this is original. The reason I say this is that it's got the solid handwheel and belted motor. If the handwheel had been spoked, it could be an indication of a model that was upgraded with a motor.
Please find that machine a home! The RAF isn't a common decal set anymore, particularly "around here". It doesn't significantly raise the value - most Singers don't command high prices, simply because they're so plentiful - but it does make it desirable to some people.