On my 1 Ton, the bolt head serves to locate the spring and caster shim to the axle. Usually those bolts have round heads. I watched a spring guy take a hex bolt and grind ir round for this purpose. The other thing is that the head needs to be deep enough to reach through the caster shim and into the axle. I don't know how the setup works on a half ton.
As far as function after it is mounted, I have seen it stated that by that time the bolt has done it's job and the assembly is actually secured by the U bolts.
