I see it has the tensioner on the face plate, so it is a model 15. The question now is age....the serial number will be on the the front of the bed in front of the main column, on a brass plate. If you can get that, we do know source to find month and year that serial number was allotted.
BTW, the 15s are very good for free motion quilting, as the feed dogs drop and the throat is very large, allowing lots of room for rolling up your quilt.