It’s planter wire or sometimes called check-row wire. Rolled in a large spool and mounted to the planter, the wire is attached to the end of the field as the planter follows the wire every time a knot or check (the part you found) is encountered the planter it drops a seed.
After looking at it a little closer and scaling off of the penny, I change my thoughts on them from a trip link from a corn planter to a lift link out of an old chain pump. The rings are rotated 90 degrees from each other and the bell is much wider than a corn planter trip link.
After looking at Vtswingers photo again creskol is correct! I should of looked closer at the link, size and the offset. I have one of these pumps too. I collect barbed wire and probably have 20 different types of check-wire and just never connected it with my water pump!