It's a Cap and a Tool
You have been using your Leica M with screw mount lenses via the Leitz screw to bayonet adapters IRZOO (50mm), ISBOO (90mm), or ISOOZ (135mm). Somehow you have jammed the adapter to the camera and are having difficulty removing it.
Do not reach for your hand tools! No pliers, screwdrivers, or prybars! Leitz produced a tool for removing stuck adapters. It is the M rear lens cap.
Tiny tabs at the rear of the cap fit into the cutouts on the adapter. Position the tabs into the cutouts and twist to free the adapter. Simple and effective. The illustrations show cap and adapter variations.
M rear lens caps showing tabs on later units. Early caps 9mm thick, later 14mm Early cap
Rear cap ready to remove ISOOZ adapter from M3. Three cutouts.
Early cap at left has no tabs. Note red dot for positioning lens.
Cap tab positioned into cutout.
Ready to remove ISBOO. Single cutout at 12 o’clock.
Cap tab fits adapter cutout.