This item is used on multiple models.
This is the complete take-apart thumbscrew assembly, #129-200. Also known on some older Penn part lists as a TA-200
These parts of the assembly can also be purchased separately if available:
130-200 Thumb Screw Bushing
131-200 Thumb Screw Spring
132-200 Brass Ring Nut
See the brass ring nut holding on the spring at the end of the screw?
That has to come off in order to install the new assembly.
Put the rest of the assembly thru the plate. Screw the ring back on as far as you can, then use something thin to jam into the hole so that you can pin that ring still, and complete screwing in the thumb screw assembly.
You would unistall it the same way, by pinning the ring, and unscrewing the assembly all the way.