This screw fits the slack regulator (part 45863) which regulates the thread by the take-up spring. Available in vintage original chrome or black. Check under the tutorial tab to know how to put the tension unit together, or if you are unsure about what piece you need.
Slack regulator is not included with screw, but can be purchased separately.
This screw fits the Singer 221, 221K, 221J, and 222 as well as the Singer 301.