Probably one of the many gadgets used to help train people to keep a consistent hold with their gun. It wouldn't need to be as precise as a rifle peep sight but may help someone keep their head/eye in a position where they're looking in the same direction as the rib is running.
A bit like the clip on blinds or blanks used to make a person use one or other eye every time the put the gun to their shoulder.

Last edited by Nick. C; 01/14/19 02:14 PM.
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