It's an axle. I have seen a lot of them in my years being part of a land survey crew. Used to be that whatever chunk of metal you could find, was used as a marker. I've also seen all kinds of pipes including copper and stainless, various steel sections like angle iron, channel and tube stock, a couple of rifle barrels, rebar, concrete forming pins, brass rods, nails, bolts, threaded rod, holes in rocks, fence posts, etc. You get the idea. State and local laws have made it pretty restrictive now, most surveyors have to use a pipe or rebar with a cap on it clearly stating their name and license number.