Hi Jodie, what a find! That is certainly an unusual model 15, and absurdly well-preserved for a machine that age. As you've deduced, it's from the early 1940s; I only own one machine older than that that is as well preserved, and it was a one-owner machine owned by a Singer sewing instructor.
The "SV" designation means the machine was specially made-to-order. There's no telling what the customization was, but I'm sure you'll eventually deduce it by using the machine.
Because it is so old, chances are the wiring for the light and/or motor will be brittle and need replacing.
The machine you have, and particularly in that condition, is really very unusual--I'm talking Smithsonian quality. That doesn't mean it's worth big bucks, as there's no governing body or official documents determining value, like there is with old cars; but if I were you, I'd certainly never let that machine go--you're not likely to find another one like that in your lifetime.
To preserve the machine, please do keep it in a climate-controlled environment--please don't store it in an attic or basement, or anywhere where it will undergo seasonal changes in temperature or humidity, or you will see the machine's finish deteriorate rapidly.
And above all, don't be afraid to use it! Singers are extraordinarily well-made machines and will last for generations (save for the wiring insulation, which will naturally deteriorate over time). You may have to do a little motor maintenance to keep it running well, i.e. swapping out the grease wicks and ensuring the machine is properly oiled and greased, and you'll need to periodically clean the lint out from under the feed dogs.
Also, don't be afraid to touch the take-up lever, as a previous poster wrote; they are impossible to break with your finger, and can only be broken in shipping or if the machine suffers a fall. I'll often manually stop the take-up lever if I'm oiling the handwheel shaft in bobbin-winding mode.
Again, congratulations on finding such a fantastic machine!