This is the motor cap screw that measures 1/2" across the bottom.  It covers the carbon brush inside the motor.  There are actually two motor caps on your Featherweight motor, because there are two carbon brushes - one is seen clearly on the top of the motor, but the other one is hidden on the underside. You can access the bottom motor cap by inserting a screwdriver into the allotted hole on the bottom of the machine or by removing the motor altogether. 

You can see in the photo above how the broken one needs replaced.

These original motor caps will only fit Featherweight motors that are Bakelite.  Be sure to measure your cap, so that you can get the right one.