I think Bob Baron made the earliest mention of the flat pitch prop that I am aware of. I think it was the early version of his Humbug that is classic legal that he ran a combat ST.35 and a 10-4 prop on. I think it was a M.A.N article on that airplane that I read that in, and he went into detail about the effect. Not as far back as 1961 but back that-a-way. I'll have to dig that one out and re-read again.
Type at you later,
Bob Barron ran the Super Tigre G-21 .35 (ball bearing) combat engine as opposed to the plain bearing Super Tigre .35 combat.