"'Precious warm walking, isn't it?' said Lowten, drawing a
Bramah key from his pocket, with a small plug therein, to keep the dust out."


Bramah NS27M (Metal Door) Dead Lock
(Reference #2115-KD43Nickle
The site where I found this shows a deadbolt, with a small lock and key to prevent its being jimmied (opened by lockpicking thief)