I do not think that this is exceedingly old.
It has surface corrosion, certainly, but if it had been in the ground for hundreds of years, I would have expected far deeper pitting.
It has an even depth, suggesting it came from a sheet. Meaning a rolling mill - post industrial revolution. It also looks as if it were stamped out, rather than cut by hand. Again meaning post industrial revolution. There are no no hammer plenishing marks, again suggestive of that time frame. There is also a blue/blackish finish under the corrosion, giving the same impression.
As to what it is, I have seen a similar thing, but with a disc at one end and a triangle at the other. That thing was used as a one way key. You have two gates - only one is to be opened at any one time. One side of the key fits one gate which opens, but the other cannot be opened as its key is trapped in the first gate.
Modern varieties are in use today.
For what it is worth, that is my guess.
Regards
Chas
