As the bolt wears the screw in a 21 is turned in to allow it to seat deeper which also allows the lever to go further left. If you use that screw to center the lever then the bbls won't bolt up tight. I din't have a Fox model B but I didn't recall they had an adjustment. They use a wedge shaped bolt locking in the top rib extension & as the bolt wears in the wedge goes deeper thus the lever comes around further. As a general rule, there are a few exceptions, on a gun with compensating bolts as the bolts wear the lever comes around further, that's how they were designed.
