I"ve had a JD 720 haybine for about 5 years, and am very happy with it. I"m not too sure the differences between the 720 and the 820. On my 720, the teeth are easy to change as they come up individually with a 7/16" wrench, so no rivets to pound out like the old style. I bought the manual from ebay just to have. I don"t plan on getting rid of this haybine anytime soon.
Hope this helps