If you have the Treadle it is a Model 15-96 and if it is Hand-Crank it is the Model 15-98. The Tension Adjustment Dial/Knob should be facing toward you as you have indicated and here is NO large Knob/Dial in the middle of the arm.Now locate and look at the Slide Plate and Needle Plate.If your machine is the 3/4-Size model with the 12 to 13 inch Bed (30cm to 32cm) then you have the Model 28.
Look at the pictures below to confirm your machine's identity.
Copyrights are owned by the Berlin-based agency "Culture Management Berlin".
A joint licensing agreement for offering this entire database on the Internet, print media is only between the Culture Management Berlin [Germany] and the Singer Corporation NV [USA]. The only production records from the Anderson factory is in 1951 when a limited edition of only 10,000 Singer Model 301 sewing machines were manufactured for the Centennial Year Celebration.
Also, any Singer machines made for INDUSTRIAL/COMMERCIAL use are not covered here at this time.
As I work out more parts to this template I will update in the future. You have indicated that your machine has the large Knob/Dial on the front of the arm.
Singer along with the Berlin-based agency "Culture Management Berlin" created a database of Singer sewing machine world productions in 2000 and 5 years later the list was finally finished and published in 2005.