Mystery Bolt

I am a bit of a Pack Rat. When I am walking around I will pick up any loose screws or nails I notice lying around.

When I was in my garage yesterday I found part of a bolt in a box of rags. I don't know how it got there, but I imagine I picked it up on one of my walks. It is kind of an unusual piece. From the chip out of the broken end, and the way the broken end is not stretched or distorted, it appears to be the head of a hardened bolt that broke under a severe shearing load. Although it has the basic configuration of a bolt head, it is not really a bolt: the head is completely round. Further, it is tapered, so it would be difficult to grip even with a pipe wrench. The head is painted black, and there are six evenly spaced hash marks around the circumference of the top surface. There are also two marks there: an X and the number 16, on opposite sides. The marks and hash marks are further evidence that this is hardened steel.

The head has a profile similar to that of a railroad wheel, but there is no evidence of wear on what would be the rolling surface.

I am thinking it probably came from a heavy duty truck, but why are there no flats for a wrench on the sides of the head?

The bolt diameter is 5/8" NC (National Coarse, which is 11 threads per inch).

Update, April 25: I thought a little more about this, and now I am thinking there was a coil spring around this bolt, there to absorb some kind of shock. That would explain the heavy duty one piece head. If it had been screwed into the frame, and a lock nut tightened onto the back side, that would explain the lack of flats for a wrench. I think it was probably from a piece of equipment like one of those portable forklifts that hang off the back of semi's, or a concrete pumper, or perhaps a tilt bed tow truck. Something like that would be more likely to use an unusual bolt like this than a regular truck would.

Update January 2017 replace missing pictures.