I'd place that bow in or very close to 1964 since a Pre -Super Diablo. One or two years they made both the 54 and the 58 inch version but I simply dont have the info anymore.
Someone here will be able to nail it down pat, I'm sure.
Nice bows, congrats on owning one I've always thought was one of the best I've ever had in the Howatt line.
God Bless