Around 1964 I owned a 1936 Ford. I was the second owner. It had been assembled using nearly every variant of screw head you could imagine...some straight slot, a few with square recess head screws, a few had two small holes diametrically opposed (which were almost impossible to remove as they always rusted/seized aggressively) and a couple of others. I don't recall any original Phillips head screws but a few had been replaced with Phillips screws (probably by the dealership or by a local mechanic). I finally decided Ford handed whatever screws they had handy to the various assembly stations and that's what they used. Otherwise I can't imagine why so many various screws were used.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end