Here are three keys from my collection. From this angle they just look like ordinary keys. They are not a set; the one on the left is much earlier than the other two and, although the others look to be the same, the collars are different. Have a look at the second photo to see what is unusual about them.
They are chatelaine keys and would have hung from the belt of the housekeeper in a large house. When she looked along the shaft of the key she was able to quickly select the correct key, just by looking at the number.