For what it's worth, on my '51, fastener types were all over the place.
Some clutch head sheet metal screws, some phillips head sheet metal screws.
Some clutch head machine screws with hex nuts, and a few with square nuts, mostly 1/4-20 or smaller.
Some hex head machine screws with hex nuts, and very few with square nuts.

Even if you're doing a concours restoration, I'm not sure it's going to matter. Who's gonna know for sure?

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.