Grant - I've written quite bit about other companies who made archery equipment as agents for Howard Hill in the 1930s and 1940s.
After the early 1950s, I really don't know much about who manufactured Howard Hill's tackle. I just happen to have one, I really don't collect Howard Hill recurves so never had a reason to research them.
It is actually kind of interesting that we have two bows so far, they are marked differently and were obviously made on slightly different forms.