As quoted from ad.
https://www.proxibid.com/asp/LotDeta...1416479274300#
The Powell Indicating Device consists of two components 1) A special left grip that has a small slot cut lengthwise down the grip with a small delicate metal strip attached to the backside that was covered with a 3 1/4 inch very fragile celluloid strip. The metal strip was inscribed with black numbers (1-7) running from the top to the bottom of the strip, that indicated the number of rounds remaining in the magazine. 2) The second part was the special cartridge counter magazine. This magazine, although similar to a standard wood base magazine, has been slotted on the left side of the body and was fitted with a special follower that had a small pin/indicator projecting on the left side. This small indicator pin has a single horizontal witness mark on top and the pin slides up and down under the celluloid strip as the rounds are fired, indicating how many cartridges were actually left in the magazine; a simple but very effective device.