This vintage original thread cutter fits most vintage Singer Sewing Machines, including the Singer Featherweight 221 and 222K. This is a part that would have originally come on the machine.
These thread cutters are not very sharp and are more useful as having a convenient edge to yank the threads against for cutting. You still need to apply adequate pressure of the threads.
To apply these to your presser bar, you will slide it on starting from the bottom of the presser bar (with the presser foot off). It takes quite a bit of maneuvering, but this is so it will not easily slide on and off - providing adequate stability to use the cutting edge.