Now that I have the pictures and concerns conveniently in one place (this thread) I called Unverferth and asked them. I sent the link to the representative that answered the phone. He had to ask around a bit but in the end he said the only purpose of the o-rings is to assist in hooking the cart to the tractor. He said after you get everything lined up you're supposed to take the o-ring out before installing the hitch pin. There are two o-rings just so you have a spare in case you lose or damage one of them.