You've probably seen the stuff in the archives here on TG; that's all I know of. I just read through all that stuff a couple of days ago myself (again). My dad was all about archery in the 60s-early 70s, and had a shop in southern California. He was friends with Jack, but I don't know the details. (Dad died in 2007.) He field-tested a Gamemaster in 1968, and was talking to Jack about doing a follow-up to that in 2005 around when Jack died. I wish I knew more details. But dad had plenty of good things to say about Jack Howard's bows and archery. After dad died, we found three brand new Gamemasters he had "hidden" in the garage!
I do have a copy of the Bow & Arrow magazine where they talk about how Jack made his bows, and where C.R. Learn and my dad went out hunting with them on Catalina Island; PM me if you're interested.
I'll be watching to see if you come across anything written about Jack.
Roman "Archie" Gray Jr.