Engineers and fitters would be issued with maybe half a dozen of these brass discs or "tool checks" each with the same unique ID number linking them with the owner. These would be exchanged for specialist or calibrated tooling at the line, hangar or workshop stores, that you wouldn't be expected to have in your personal toolbox, on a day to day or shift basis and returned when the tooling came back into the stores when finished with. Stores staff could tally which items of tooling had gone to whom and at the end of the check or maintenance input it could be determined if any were outstanding.
Nowadays the same process is done by barcode or RFID chips.