I don't think this was an excavated find; the guy i got it from said this was included in a lot with various things (weapons, i gather).
These being humble pieces and not those belonging in nobility armour, are more subject to weather exposure and to the places they were abandoned, specially the side that was facing the ground. There is a similar example in the Barcarena museum which, together with a more refined one, with Switz marks and Swedish re-marks, were found by a sports diver near a place where actually XV-XVII century wrecks occurred;
only that, judging by their condition and knowing that iron corrodes fast in salty ambiances, they didn't reside there longer than a couple decades. If only that couple could speak.
The last picture here uploaded is of yet another example, this one belonging to a private collection. In this one you can see again the strap fastenings, typical of pikemen cossoletes, while in the one at the museum, the text refers that only some residuals remain.
You will also notice that all these rustic plates were not made following the finest design; instead care was taken to make resistant.
As for your wise advice to preserve mine with some oiling, that was the last of a multi episode saga. When i noticed that the varnish would easily scratch with a fingernail, i first rubbed it with a very soft brass brush. Lots of varnish 'powder' came off, but not the whole cover.That's when i gave it a double rubbing with acetone; then washed it with water & soap; and after pass it a gentle layer of 'sewing machine' oil.
BTW, bizarre comparison, that of an equipment for removing mines
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