The more I look at it, the more I'm not sure about it being a model 15. All of the model 15s that I have seen have the tension midway down from the top. From your picture, this one looks like the tension dial is at the top.
A good place to ask is at the Yahoo group...
http://groups.yahoo.com/group/vintagesingers/
They have a lot of pictures of older Singers in their photo albums.
I love your find. It is really interesting.