Jason in Enid is correct. Way back when I was a youngster (1950s/60s), rectangular flat cans of sardines, anchovies, etc came with that kind of can-opener "winding-key" attached to the can. The can's top had a short tab of metal projecting from it. You would insert the tab into the slot in the key, and turn it like you were winding a clock, which pulled a strip of metal loose from the can's top, opening it.