Well... I found this... I'm just not so sure how the stem doesn't get depressed into the powder and/or cause overflow in cases that are pretty full with powder...
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Notes:
This device resembles a standard reloading die with a moveable stem in the center. It is threaded into a station in a progressive press at a point just after the case is charged with powder.
When the charged case is brought up into the Powder Cop during the next operation of the press, the center stem in the Powder Cop contacts the powder charge in the case.
After adjustment of the Powder Cop, when a correct powder charge is present in the case, a white O-ring will be visible on the Powder Cop center stem and the O-ring will be even with the top surface of the Powder Cop body.
If there is no powder charge in the case, the stem will not rise.
If there is an overcharge of powder, the stem will lift the O-ring well above the surface of the Powder Cop.