It appears to be what's commonly referred to as a Japanese 15 clone. It's a copy of a Singer 15. They were manufactured after WWII in Japan.
Most are very reliable machines, although Miriam says that some of early ones sometimes need some timing work. They are all metal and will last forever. I've had a couple of the later Japanese machines and they worked well, although one was really noisy.
Usually, you can just oil them and start using them. Singer feet and accessories fit them, so they are inexpensive and plentiful. You can get a generic manual for free at ismacs.
bkay