I would have to see the serial to be certain but singer had a 15 that was a green before they came out with the newer body style like this one shown.
But I would be willing to bet the farm it is a 15 just from a few of the features I can see from the photo. The first thing is the location of the bobbin and the tension assembly. Then you can look at the stitch regulator and the way the motor is attached.
Billy