earlwb

Way back then the common propellers to use on the .60 or .61 engines were 11x7, 11x7.5, 11x7.75 (seven and three quarters) and the 11x8 sizes. 12x6, 12x7 and 13x6 props were used too. Usually the engine had to be really well broken in good to run the bigger props though, but it depends on the engine brand though. The engines that liked to rev up more used the smaller 11x7 or so props and the other slower rev'ing engines used the higher pitched props (11x8). But I remember quite a few guys running the 11x7.5 and 11x7.75 props a lot, myself included).
The 12x6 props of that era were quite popular on the engines too. But back then it was Top Flite, Rev-Up, andf Zinger mostly. We didn't have fiber filled plastic props at the time, the nylon props were quite questionable as to whether they would come apart on you or not.

I haven to have a old Fox .60 Blue head engine (with the dual plug head on it) and it is using a 12x6 prop at present too.  I used to run a 11x7 prop on the Fox Hawk .60. But with a Fox Eagle .60, it tended to like the 11x8 props.  If I remember correctly I used to run a 11x7.75 prop on a K&B .61 a long time ago.