The Hatter introduced in Carroll's Alice's Adventures in Wonderland wears a large top hat with a hatband reading "In this style 10 / 6". This is the hat's price tag, indicative of The Hatter's trade, and giving the price in pre-decimal British money as ten shillings and six pence (or half a guinea).

What is written on the mad hatter's hat? Let's have some stream of consciousness first.
The relevant information to answer the above question is: The Hatter introduced in Carroll's Alice's Adventures in Wonderland wears a large top hat with a hatband reading "In this style 10 / 6". So, the answer is In this style 10 / 6.