turtles

Hi,
Please, can anbody update or correct my memory:
I faintly remember having read that in acient times among printers there was the tradition that up to three errors in a book would be regarded as "okay". The nickname for an error was "turtle" or, since I think the story took place in Italy, "tartaruga". Therefore a printer created his mark showing two turtles so that in the rest of the book only one more turtle (= mistake) could be expected. In doing so he referred to his superior quality.
I cannot find this story on the web. Is my memory wrong? Has anybody heard of a similar story? Thanks in advance for helpful suggestions!

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