Hiker, these are a mystery indeed. I'll take a wild guess though based on a few particular things in their design.
1) the bolts are short, so what went between them was fairly thin.
2) the hole to the far right looks like the pivot point. (lever pivoted on this hole)
3) the hole next to that holes the bolt head flush on the inside.
4) the two semicircular depressions, coupled with the sloping side of the relic indicate this was where an angled tab would be locked in place, and which could be easily unlocked.
Based on the above, my completely wild guess would be that these objects (4 pairs?) were mounted on the bottom of four levers which were part of an early four bottom plow, cultivator, or harrow; and the levers raised or lowered the working parts. The semicircles would have clipped the apparatus in the up position and locked it there. As you found at least one bolt without a nut, my guess is these were not mounted on wooden levers (too weak, and as well, the bolts with nuts would have remained after the wood rotted away), but instead were mounted on steel and were for some reason disassembled. (most four bottom cultivators in use in the 20th century had a different spring loaded arrangement over a toothed piece of metal to raise and lower, which is what bothers me about this guess. It could be earlier though)