What does a letter mean in a newspaper, magazine or letter declaration?

That is, wooden slips

Also known as woodcut, woodcut or woodcut, it is a form of books made by writing words with certain specifications of wooden boards (a few with triangular wooden columns). According to records, there are several specifications of wooden slips, but the most common one is 1 square foot, and its thickness is 1 inch, which means "one foot and one inch", so it is called "ruler". Mostly used in calligraphy and painting.