Weston:  Well done you.  You Nailed It!  I suppose a little dab of gasket seal, lock-tite, or rubber cement would lock your replacement shaft in place but allow easy removal later.  You could also file a screwdriver slot in the end of the shaft to allow easy positioning and removal.  The Cutoff is remarkably simple and easily watched with the side-plate off.  When the lever is up (on most models) the flat on the shaft is 'down'.  This fills the slot/hole that is machined in the receiver and allows cartridge rims to smoothly slide past.  When the lever is 'down', some of the round part of the shaft blocks the top cartridge (rim) from sliding through.  If nothing is in the cutoff slot/hole, a bit of a rough spot exists that creates a 'bump' in the feed.