JmeB welcome to the site. I'm not familiar with the 615U in particular. I'm not sure if the 615U is a touch & sew. I don't think it is but it was made at the same time and probably shares a lot of similarities. I just picked up a 648 touch and sew missing a few parts and found this place:
http://tandtrepair.com/index.html They seem to deal a lot with machines of that vintage and may be familiar with your model.
Miriam, I just saw that post. I don't wear ear rings but if I buy more metal ones I'll keep you in mind.
Rodney