Hmmm. Good question. I searched and came up with http://www.bobcat.com/historical_specs/ which I suspect you may have already found. Only difference I could find there the 743 went into production in 1981 and the 743 DS in 1983. Then, on the specs page: 743 Pump Capacity 11 GPM Auxiliary Std Flow 11 GPM Auxiliary High Flow N/A 743ds Pump Capacity 11 GPM Auxiliary Std Flow N/A Auxiliary High Flow 11 GPM The auxiliary high and standard flows are reversed. But they're both just 11gpm, so it's not like high flow was added. Maybe that's just a typo. Guess I'm really just adding to the head scratching. I'm surprised none of our pros on this board have answered. Regards, Marty