This is the dedicated push-on cap for the lens hood which is originally supplied with the 21mm f/1.4 Summilux-M Aspherical lens, and is offered here as a replacement for a lost or damaged cap.
This cap's specifically made for the proprietary hood for the 21mm f/1.4 Summilux, and isn't for use upon any other hood. It has a classy embossed Leica logo on the front. The 21mm Summilux's hood accepts Series VIII filters on the inner side (lens side) of the hood.
Made for Leica's 21mm f/1.4 Summilux-M lens