Sorry Jeremy, In my rush to provide you with pictures, I didn't notice yours is a plain bearing ML10 so probably best to ignore my pictures & instructions above.
But for the suspect setscrew at the bottom of your small pulley groove…..not sure how different your version of the ML10 is, but on mine the pulley cone (and small gear permanently locked to it) turns freely on the spindle whilst the big bull gear is permanently keyed to the spindle. There's a radial sliding toothed peg which locks the two gears together for normal direct drive, and unlocks them for reduction drive through the backgear.
So if you have a similar feature for backgear disengagenment, you could set it to disengage, and that should allow the pulley cone to spin. If it does, you'll know the set screw you drilled to look for isn't there/isn't gripping the pulley cone.