Danged if I know. Maybe a 1 mm pitch, fine thread, instead of 1.5? Thought they had to be metric. Everything else on the truck seems to be.
SAE 5/16 was tried but maybe not fine thread.
There's plenty of non-metric on the truck. One thing that comes to mind immediately is the lug nuts, and another that I just did last weekend is the u-joints - the cap bolts are 5/16" x 24 (I chased them all with a tap)..... pretty much everything on the axles is SAE now that I think on it.
There's a LOT of 10-32 all over the place on my M1079 shelter. The factory used the hell out of 10-32 rivet nuts. That's size #10, 32 threads per INCH.
Some of the confusion probably stems from the cross-over nature of some of the hex sizes used for fastener heads - 1/2" and 13mm, 9/16" and 14mm, 3/4" and 19mm being often interchangeable without noticing.
The cab and some of the other bits are licensed from Steyr and thus are very much metric. So there's a mix. But PLENTY of SAE still evident.