Featherweight is also a low shank. Don't know about a 66. I love my 15-91. It will sew when everything else fails. I have a treadle but have not used it in many years. Learned to sew on one when I was a kid. But use my Janome4800QC and Singer 403 now. 403 is a slant shank with zz stitch but I still like the 15-91 better. The feeddogs will drop for FM and it is heavy so it doesn't travel when you sew on it.Have you checked the vintage Singer groups on Yahoo? They have information on all the vintage Singers. They have been a big help to me over the years with my vintage Singers. Hope you have good luck.