The one on the right is a stock 600, vacuum secondaries. It has a slight vacuum leak at the throttle butterfly shaft so will probably need a new bushing there as well as a rebuild. The one on the left has the air horn shaved off with a vacuum secondary spring kit and the check ball removed in the diaphragm housing. Holley tech support said that this will help the secondaries open faster and WOW WERE THEY CORRECT! It was a blast to drive, but because this was on a part time daily driver, I removed it as spinning tires in rush hour traffic wasn't good. Both or these carbs have been hanging on the wall for a few years so a rebuild is definitely needed. $40 for both and local pickup. Thanks.