As MimiBug123 said it's a Japanese 15 clone. A close copy of the Singer 15-90, not the 15-91. They were imported by the gazillions after WW II and most are of excellent quality.
There are generic manuals available on line from several sources, but your Singer manual will cover the basics.
They are low shank machines so any presser foot, or attachment suitable for a LS straight stitch machine will work on them.
My mom had a HOTHER, a 15 clone, for eons. I have that machine. Like yours, it still sews beautiful stitches.
Joe