Adjuster nut, push rod 5/16" UNF shouldered

On these later machines the rear brake setup uses a push rod running to the master cylinder, and this nut is what sets the rod's position, which in turn sets the free play and engagement point of the pedal. It's a simple, low-cost part but the kind of thing that's easy to lose track of when a rear brake is apart on the bench, so it's worth having a spare on hand.

The shoulder on the nut helps it locate correctly against the clevis or pushrod end rather than just cinching straight down flush, which matters for keeping the adjustment stable once it's set.