Can't make out if I'm looking at the same side in every photo, as it takes forever to get a larger image to happen. (got to love thumbnail pictures) So by quoting your post I'm hoping it will enlarge them up a tad.
Sometimes camp lead that falls to the earth while still molten will take on strange charactoristics from grass imprinting.....don't know if that's what you got or not though I have yet to find a piece as rounded and flat as yours.