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For years I've had had a jar full of a nicely labelled M38 part listed in Group 1805: Nut, plain, clinch, square-pilot, 3/8-24NF, H101-01-43562, 143562, 1 required for the front floor pan. The nut is in an 11/16" square cage and it sharply tapers to a narrower empty square cage 1/2"across. It was a mystery to me until I was changing rubber body shims on a nice original M38. The left No.3 body cushion has 3/8-24NF bolt going upwards into a nut imbedded the hat channel (the bolt can't pass through the floor because the gas tank floor is right there). Some rust in the area prevented me from seeing the exact detail but I am pretty sure this is where the clinch nut goes. I assume it was welded into a hole in the hat channel, providing a captured nut for the body mounting bolt. Now I have to see if the repro hat channel kits incorporate this nut. If your hat channel is rusty there then a repair is very difficult without replacing the hat channel. Incidentally some of the 3/8-24NF body mounting bolts are listed in the ORD9 a being 2" long: they in fact measure 1-3/4" long and Ryan Miller's book also says they are 1-3/4" long. Is the 2" length in the ORD9 a previously recognized mistake?

My floor pan is rusted and damaged/missing in just that area.
The bolt is there through the hat channel with the nut on the top
side.

Yes, there is a square hole in the hat channel that looks like the
tapered nut should sit down into and welded.
Brian

I'll bet that many times when a restorer tried to remove that bolt going up through the left outrigger the caged nut broke loose up inside the hat channel. The quick solution would have been to run a bolt down from inside the body and through to the outrigger. But this would leave an exposed bolt head that could rub against the bottom of the fuel tank. Related to body hold-down bolts: I couldn't find a single M38 or M38A1 at the Topeka MVPA Convention that had the correct low-profile "square-neck bolts" (look like carriage bolts) in the 2 floor holes near the front edge of the rear seat. They are hard to find: the head is 1" diameter and only 3/16" thick: not illustrated in Ryan's book either.

Mine are there, but I haven't reached that end other than tailgate
and surround work. Like many other OEM bolts, I fully expect them
to snap right off at the slightest attempt at removal.