I have a Two-Digit Dating System for Featherweights that I got somewhere off the internet. This is the information I have for the serial number you've given.
...221__AF 86-89 ….. 04/01/41
221__ AF 99 ……….. 07/01/41 *****transitional production run
221__ AG 00-01 ….. 07/01/41...
So yours should be in this period of time:
"221__AF 99 ……….. 07/01/41 *****transitional production run"