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"This 14th century door at Exeter Cathedral, UK, is thought to be the oldest existing cat flap
A cat was paid a penny each week, to keep down the rats and mice in the north tower, and a cat flap was cut into the door below the astronomical clock to allow the cat to carry out its duties.
Records of payments were entered in the Cathedral archives from 1305 to 1467, the penny a week being enough to buy food to supplement a heavy diet of rodents."
They explain it in the next paragraph. Apparently they used it to buy food.
That being said, I don't see any citations here so it could all be bullshit.