Photo below shows how CC plug converts an open center control valve if it's the convertible type valve.
TX Jim, That really does help. I sent a message to the seller to let them know that I was interested but I had to figure out how to adapt it to the JD 2840 closed center hydraulics. They were kind enough to respond and let me know that I did need a closed center control valve to make it work. I did some searching and ran across the CC plug and wondered if that is all I would need to convert it. Otherwise replacing the whole control valve could add some cost to the purchase. Thank you for that information, I know what to research to see if it can be converted easily. If not, I will look for a similar model that supports that. Thanks!