SerendipiD, I think all the Singer 400 series except one are ZZ machines as are the 500s. Necchi had the very first one. My Elna is another. shopgoodwill.com just sold the twin to my Elna for $36. Right now there is a 201 for $26, but it's not a ZZ - just advertised as Singer's finest. They also have a very old machine that needs tlc for $12.
BTW, your 15 is beautiful.