This is very interesting. My Grandfather was a cobbler and plied his trade in Liverpool. Most people wore hobnail boots and some didn't wear anything at all. Shoes I mean.
It was the custom back in the 1880's that the men went out and got drunk on about every occasion they could. Mmm, things have not changed a lot
Anyway, enough of this digression. When the men came home after a good night on the beer and gin, they invariably lost their door keys. It was then the custom, to kick the door in with his big heavy boots. After all, he did live there.
Over the years, the housewives got so fed up with their doors getting kicked in that by pure accident, one housewife discovered that a nail that had fallen from her husband's hobnail boot, after demolishing the door, could actually open the lock on the door. This was an amazing discovery. Quite on par with the inventions of the Steam Engine and the Telephone.
So whenever, a guy went to get his hobnail boots repaired, he always asked for a spare nail. The rest is history. Hope this helps.