So here I am back home. The offending dust cap didn't last the first forty minutes so I tossed it in the back of the truck and continued home.
On inspection this morning the wheel/hub assembly had a lot of play. Maybe that vibrated the cap loose? I was raised 'finger tight and then back off to the first split-pin hole'. In this case that lead to about 3/4" of play at the tread.
So now it is turned on firmly finger tight and pinned there.
Off again tomorrow for a four hour trip. We shall see....
Stay tuned.
Stu
PS
to answer j-d's question; the HUB sheared about halfway between the outer end and the wheel flange. Pretty much left the outer bearing and races intact and relatively undamaged.
Drove to a parts store, ordered a complete hub including bearings, etc, bought a tube of grease and was ready to go a couple days later.