The carb you show is for pre-1971 (+pre-1970 California) applications. It has an open/atmosphere external bowl vent, no provisions for a ducted bowl vent to the charcoal can on your '72. Especially with today's alcohol-diluted gasoline, You will lose fuel (and therefore fuel mileage) to evaporation whenever nobody's foot is on the accelerator, whether the engine's running or not. See here. If I were bound and determined to run a Holley 1920 and wanted an Economaster unit, I would hold out for the one that's correct for your '72, which is a 7586.
A couple other reasons I would steer clear of this carb unless I had absolutely no other choice:
1. It's not a spring chicken, and it's got stickers and casting discoloration indicative of having been "remanufactured" in the past -- which frequently spoils the carburetor irretrievably, and
2. if this is actually a truck carburetor, it may not have a car-compatible throttle lever. Some of the truck/van applications required a different throttle lever than was used in the passenger car applications.
Ted, there are no dual-stage accelerator pumps—the accelerator pump's shot is proportionate to the amount the throttle is open. I think you meant to say 2-stage power valve (Holley calls "Economizer"), which yes, all the Economaster 1920s have—along with all the production 1920s starting in 1970.
If I had a 1972 Scamp and I wanted to put the best possible 1bbl carb on it for driveability and fuel economy, I would not hesitate (heh, see what I did there?) to make it this one.
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