The Singer 221/222 Featherweight and other Singers use a ring connector crimped to the wires which attach to the back of the Singer 3-prong Bakelite terminal block / plug socket. You also find these ring connectors on the wires that attach to most of the vintage foot controllers.
This set allows you to recreate the original Singer ring connectors so that you can safely rewire to the back of the Bakelite terminal block. Simply strip the wire, twist it into a circle around the ring while it is in the tool, and squeeze hard on the handle. You will be pleased with the ease and result of using this tool.