In the contest of the whirlybird seed, those are pretty small anchors. Probably from some sort of rowboat.
I'm not feeling the ballast theory. I'm also not feeling the "didn't pass QC so it was dumped overboard" either. I don't think bad castings would have made it out of the furnace, and even if they did get to the boat, it's not likely they would have left land. If it did get on the ship, it's not the captains job to QC the freight and dump what he didn't like overboard.
They could be scrap pieces that were being transported to a forge to be reclaimed but that doesn't explain why they'd be in the river.
This is a real mystery.
Jerseyman, do you know if any journals exist of any of the iron transporting ships that plied the waters of the Mullica?
A nice mystery is where did all of that pig iron go?