The word "cap" was not the result of having to give a name to a physical object: cover, slice, etc ... but the need to provide an action name. You could say: close, cover, disguise, conceal, muffle, protect, overlap, cover ... hunger.
Food portions not begin selling in the pubs, because in them there was no more wine. They began in convenience stores, small grocery stores where they began to dispense wine for the street, as one of his many articles and where, by extension, customers will be served wine at retail. Hence to place them a slice of sausage on the cup or glass, no more than a step. As anyone sold there saucers or trays they could not put the pieces anywhere but on the receptacles (The historic taverns folder. Manuel Maria Lopez Aleixandre).