Frans, you're recalling the early Sohner Trucks. Those chippers were Fitchburg's. They were actually good machines. Chuck and duck, mind you, but could take brush as fast as 2 people could throw it in. All providing sharp blades and a set of matching belts off the PTO.
Anyone know about those Pro-mark chippers. Looks about the same as a Vermeer 6". Disc looks to be 90 degrees to the throat. What engines on those? How old?