Certainly a Closed Center Tractor.
I want you to look at the tractors Transmission Filter Housing. (Under your right foot, bolts up to the bottom of the trasmission case, 3/4" wrench required) Most of these housings have a threaded port on them. Is there a hydraulic hose running to the port?
If yes, then you should be able to run an open center valve without hurting the tractor. If no, then after running it for a short time, the cylinder can start "Jumping" this is really the main pump running out of oil, and is very hard on the pump.
Anyway, it is aClosed Center tractor, and you should switch the valve over anyway. "Most" Splitter valves, the "outlet" from the splitter valve is 3/4 inch pipe threads. If you take the Hose off, and the Fitting out, you will see either A) a Pipe plug, or B) a threaded hole that might take a 1/2 " pipe plug.
If A, you are already set up for Closed center, rock on. If B, then put a pipe plug in it, then you are set! It is that easy!
Also, Here is some technical Info on the tractor.
http://www.tractordata.com/farm-tractors/000/0/6/65-john-deere-1020.html