My guess is a 115. At first I was going to tell you it was a 15, but the thread tensioner looks wrong for a 15. Ismacs has the correct manual:
http://www.ismacs.net/singer_sewing_...e-manuals.html
The one I downloaded from Singer was wrong, it had the right model number, but showed a machine with a tensioner facing you, which is clearly not right. There must have been 2 model 115s at some time.
If I could see the bobbin case, I'd have a better idea if it's a 115, but it looks like mine, except your decal set is different than mine.
The C series serial numbers were destroyed in the war, so there's no record from Singer about it, I can't get you a model from that, or a date.