[heikkilevanto]

With all respect, I call bullshit to your "4 unique keys" theory. Locks have a long and varied history all around the world. There is no way a single skeleton key would even fit into an Arabic, Chinese, Indian, Venetian, Swiss, or German lock.

[bluGill]

For practical purposes you are wrong. In any given city 4 door locks is about right. There are padlocks and other types of lock, but the subject here is door locks in one location so4 is close enough and was the truth in the US where my grandparents grew up.

Until not long ago all locks were bad