There are lots of info on these on the web. If you search for " model 15 clones", "Japanese 15 clones" or similar something is bound to turn up. After WWII the Japanese made a lot of sewing machines modeled on Singers original model 15 (later version with the stitch length lever and reverse). They gradually added a few new features like feed dogs settings, and sometimes a hinged bobbin lid. By the 50s they gradually developed their own zigzag machines too, we know them as Janome and Brother today.
Here is a link with some info, and
here is another. I'm guessing yours is quite early as it's identical to a model 15 with the stitch length lever and a belt driven motor. Sometimes these machines were imported with out any electrical parts and fitted with US made motor and lights. Much the same happened in Europe too. Some like these machines a lot, with others they are not the favorite, but trust your own judgment once you know the different between the brands.