I think there is a 1901 and a 1902 Edelbrock that are the 500 CFM that one is electric choke and one is manual choke. But then the 1903 and 1904 are meant to run together as dual quads and they are electric choke and manual choke but are calibrated a tad different specifically for a dual quad setup. so as I was researching this kind of stuff and trying to figure out the best way to calibrate my Street Demon dual quads I took a note from their book and used a larger step in the metering rods in the front carburetor... This was what finally cured my at speed tip in issues...