I have the same machine! It is indeed a 15-91. I used it to stitch very thick leather pieces together to reupholster a table for my chiropractor last week. 2 pieces of leather together was 1/4" thick!!! I just love this machine. I call it the "tank". And now it's back to sewing quilt fabric. What a versatile machine! I got mine Free on craigslist. It has a beautiful table.