Ube ... I have one like yours. Mine is a 115. There are a couple difference, but one I remember is that the 115 has a full rotary hook and the 15 has a oscillating hook. Open her bobbin cover, give the hand wheel a spin and see if the hook makes a full circle around the bobbin case. If so ... you've got a 115. If it turns half way then goes back in the other direction ... it's a 15.