Toronto. Dust and dirt everywhere! How can you keep it out of camera bodies and lenses? Easy, cap the openings! Since the early days of photography, caps have been used to keep out dust and dirt when the camera or lens is not in use.
Caps have been made of many materials – leather, paper, metal, plastic, etc. Most are signed by the lens maker but others are generic and unsigned (or signed inside and hidden from casual view). Most caps just push on, but some screw on and others actually lock in place.
These caps are often lost. Or are left behind. Or fall off. Most makers offer replacements as accessories for a few dollars. But caps change over time, They may have the same diameter and depth to fit a lens, but a different shape (shoulders, cupped, etc.) or material may be used. After the M series of cameras were released by Leitz, the company offered a conversion ring to allow old lenses to be used on the new M mount cameras. The rear lens cap for M mount lenses had three tabs added to the outer edge allowing the cap to be a wrench to remove the 1mm adapter ring from the Leica M body. These tabs were dropped years later after the old lenses were rarely used on M mount cameras and the adapters were no longer sold new.