Is the tension assembly on the right front or on the nose of the machine on the left, on the faceplate? The model 15 is on the faceplate. If it is on the right side, it could be a 66, 99, or 201. A round bobbin or shuttle? We would presume round, for a more modern machine. A 66 is heavy and big. A 99 is 3/4 size of the 66. A 201 is a machine with a tubular grease fitting on the top, right of the column and tension on the right side. Try looking on ISMACS to see the pictures, or even better, try Ebay. You will find it there for sure. After you get the serial number, you can look it up on ISMACS or singerco.com.