that is a very old gage for determing the exact maximum measurement for bullet seating depth on a given chamber in a specific caliber.
this allows a diligent handloader to set over all length so the bullet ogive is close to the lands in the bore.
a much better version can be had from hornady
a good video explaning how its used.
the basic procedure is the same for the tool in your pick. i would recommend against using the tool in your pick as it looks like some key pieces are missing.