I love the art nouveau decals on the Exelsior. You have to let us now more about it. I've noticed some of the old machines are surprisingly nice to work on and very capable. I'm in Norway, and most of the Singers that turn up here are made in Scotland, but a few German and French have turned up.
The closest I have to your 15B, is a beige 201K23. It's a different model but it has something common in the shape of teh body. My 99K had the same cylinder shaped capacitors near the plug as your 15B. They went out with a bang when I was sewing, it's years a go now but I still remember it. Luckily they managed to rewire the machine at a local repair shop, and it has behaved very well since. Be aware, they almost always need replacement on 50 year old machines.