Here are some photos of some original 1970's Boler 13 rock-guard hardware.
This will give you an idea how it works, but I don't know that it's anything to crow about. Since it's just painted mild steel, it tends to rust/leave marks.
There are two identical sets of hardware; one on the driver's side and one on the passenger side of the rock guard, about 5" or 6" in from the side of the guard.
On the top, there is a fitting that's riveted to the trailer body that is basically a flat hook, curling up. Then in the same place on the top of the rock guard there is another fitting that is basically a slot, riveted on. Thus the slot fits over the hook and the top is held on by gravity. It's not a hinge, per se, and you can lift the rock gaurd right off the trailer. Here is a photo of these two top pieces of hardware, interlocked as they would be with the guard on the trailer:
At the bottom of the rock guard there is an over-center type of clasp, something like there is on a ski boot, and then there is a "slot" fitting on the trailer shell.
Here are photos of the clasp, from front and side, and of the lower body slot.
So you lower part of this clasp until it hooks into the slot on the shell, and then you "snap" it up until it goes over center and "locks."
I don't believe there was any special, dedicated hardware provided to hold this style of rock guard up. I suppose one was just meant to take it off and set it aside. That said, two dowels or sticks or what-have-you will easily prop the rock guard up by catching the trim under the bottom edge (now up in the air and out away from the trailer) and then making a 45º angle down to the belly band.
Some people have used arrows for this, and others have made fancier props (Joy, I know, for one).
I wouldn't go out of my way to get this particular hardware, as I mentioned above, but knowing what it looks like should at least give you some ideas.