The wear pattern on mine seems to indicate that the damage is being done when the throttle is closed. Pulses from the engine rattle the slide and plate back and forth in the carb body and at small throttle openings, the bottom of the slide plate is trapped in a slot
. My humble theory is that the bottom of the plate is held snugly in the slot while the rest of the plate and slide is free to vibrate back and forth.This kind of wiggles the bottom corners of the plate and they crack off. With the O ring in properly there is a bit of cushion, backwards you have metal on metal. At 1/2 throttle
everything is free to move together. Did the bottom corner/s brake off your plate like mine? Looking at your slide plate does the wear pattern seem to match the carb bore when the throttle is closed?