Full marks for effort, major deduction for execution, though.
Personally, even if my cat loved medieval history (he doesn't), I'd still actually put a picture of him on a "lost cat" poster. I mean, if you see a ginger cat wandering Cambridge, are you just supposed to go up to it and start quizzing it on the Battle of Hastings and call the number if it responds?
On the other hand, I'm sympathetic to the need to advertise the cat's proclivities, but really, if you need a pictures of medieval cats (and yes, I've just used the words "medieval" and "cat" in the same sentence, fully knowing the risk of reigniting the flurry of cat-print-on-manuscript pictures circulating) there are much better options available.
Nevertheless, I do hope these folks find their cat.