I think the whole tractor is still OEM. There might be some aftermarket stuff made for the JD310s - Deere certainly has built a bunch of them over the last 30 years. JD310/410s are everywhere, and most still in use. I don't know about aftermarket because I've never looked! The dealer is just too handy. How is that for being a captive owner?
Anyway, the same gas spring fits both doors on about every JD cab ever made, and would be a real good place for some innovative person to market an upgrade. The gas spring installs in 5 minutes with two bolts.
There is just no logical reason to need as much force to close the cab door as it takes to open and hold the it open.
Every operator I know would buy one. Fighting with that door a few dozen times a day gets old. Someone could sell them right there on the job site off the lunch wagon. "Gimme a couple burritoes and one of those door spring things"
rScotty