Early 80s . Not sure what all was even available then. He was a 1-man operator so he got the cheapest thing available, used.All of the drum chippers I've been around had an instant-drop feed tray that prevented further feed of any branch. When the tray dropped, the wood didn't get pressed into the bed knife and all the brush chipping came to an abrupt end.
I suppose there are plenty of versions that didn't have that, but I never used one.
Before that we were loading brush into his truck and using tree saws to cut down the brush...in the truck