Chain stitching on the 503! You got the plate? I bought a Willcox and Gibbs electric chain stitcher, but it's not working and I haven't gotten to futzing with it yet. The guy I bought it from said his wife sewed with it when they first got it, then they put it away. It needs de-gunking and maybe motor work. It was very inexpensive so it'll be a fun project.
What you wrote about the buttonholer reminds me that I knew someone, Joan Kiplinger, who bought a 401 new and there wasn't a Singer buttonholer available at the time so she had to buy some off-brand. We used to trade vintage sewing stuff like bias tape and notions and she always wanted to send me the buttonholer because she hated it. So I'm thinking that Singer initially figured that people would make buttonholes with the zig zag stitch and not need a buttonholer. But people must have clamored for it.