^ to fully overhaul any derailleur of this vintage type, you first remove the stop screw.
if you try to separate the pully cage from the body without first removing the stop screw (which will then allow the release of pully spring tension), the spring will snap, and you may not later know the exact notch the spring was in originally (if there are more than one). it's nearly impossible to recharge the pully spring and reassemble the derailleur with the stop screw in place. it's the first to unscrew on disassembly and the last to rescrew on assembly.
a pully spring snapping your thumb in surprise attack is not fun.