The rubber boot was only used on the Pontiac Ram Air carburetors, not on any other early Q-jets.
Those airhorns were also cut deeper and wider to accommodate a metal retaining ring that is driven in to hold it in place.
They were NLA for quite a few years but are now being made again aftermarket.
They also came in many over the counter rebuild kits and the pump was cut for them as well, even if the kits were not being supplied for Ram Air carburetors. This probably created some confusion as I've seen a lot of carbs show up here with the cut pump shaft and rubber boot on them even though the casting wasn't cut for the retaining ring.......Cliff