can i marry your brain? lolThe tin & key combination was kept for sale to civilians after World War 1 when tinned corned beef was first seen. Early versions of the foodstuff were produced in massive quantities as a cheap food for the soldiers and was packaged in that way to survive the demands in the trenches. It was a self-contained meal that didn't require any other tool - other than the key provided - to open it.
The question of why the key - or metal strip - breaks off half way around the tin is down to poor handling technique. It is important to keep the wound strip of metal in line as it winds onto the key or it will break.