Noticed this in Dover Castle's hands-on section last weekend. Sorry about the bad picture, a difficult contrast job. Anyway the fun bit was that you could pick this lock with your finger through the keyhole. Unfortunately it's been allowed to seize up (someone has half-inched the key) so I couldn't get the bolt to slide across, but it was amusing none the less.
For those whom this has worried, there's a similar lock on the church at Dover Castle, this time mounted on the inside of the door. I couldn't get my finger through the hole in the door to reach the lock, so that's ok!
