I came across this unusual upside down T-shaped keyhole on the door of a Victorian townhouse in Glasgow today. It's for an Odell Night Latch Key, of the type shown on the right. Patented in 1792, they came in a wide variety of different shapes and you put them through the cross bar of the T and then lifted them up. They'd only open the latch if their shape fitted the inverse shape of the mechanism in the lock itself, which sat on the other side of the door.
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