I have a 3 decade old Skeet's Bore Gauge. The sizing ring is .700" diameter. That ring fits somewhat loosely on the end of the gauge, so the smaller plastic ring was supplied to keep the sizing ring from falling off of the gauge when carrying it.
The 2 head gauge I purchased through Brownells came in a box that had provisions for storing the calibration rings and the second head and storage ring. The storage and calibration rings are very close in size. The calibration rings were normally kept in the box since you generally need them only when switching heads. I have always presumed that the protective rings were intended to store the unit with the balls off the seats to prevent sticking and rough action.
If you ever manage to "disassemble" one of the heads, you have a tedious job ahead. With the way the balls and actuator fit into the head, the only way I could get it back together properly was use grease applied with a toothpick to hold the balls in place. This followed by insterting the polished balls and rod. Not something I ever want to do again.
