Peering Through A Keyhole

With old style locks “ward style” the door could be locked or unlocked from outside or inside the room. The key activated the same mechanism from both sides so it had to plunge past the mechanism from both sides. This overlap of key position meant that there was a hole all the way through the lock to allow the key access from both sides. You could look through this hole into the room. The view was limited to what was in line with the keyhole.