Hi Dick
The hole is for a bar (3-jaw chuck key handle from lathe) to enable the chuck to be unscrewed from the nose spindle. The hex bolt in the end of the clamp has been machined to suit the dome head of the rivet. If it was flat I was concerned it could distort the soft copper rivet head. This bolt can be removed and other M6 screws can be modified to suit the head of other screws, bolts, rivets etc that need machining. Hope this is the bolt you mean if not please ask again.
Thank you for your interest again
The Home Engineer
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