I have to admit that I know nothing of Curtis, nor had I made the connection between the two covers mentioned above. However, I do know of one other cover signed by Curtis from the same publisher in the same period, so we have two, possibly three examples of his work, which I'll post below.
If anyone can add anything, please drop me a line.
Four Square Books 993, 1964, 143pp, 2/6. Cover by Curtis
Four Square Books 1166, 1964, 320pp, 5/-. Cover by Curtis
Four Square Books 1250, 1965, 188pp, 3/6. Cover by Curtis?