The car I got it from was in the junk yard, and looked pretty original, but who knows.
It ran good on the V6; better than the Holley 390. That was 20 years ago though. Even 500cfm was big for a 225 V6 with a slightly bumpy cam. That was part of why I went back to the stock 2bbl, and partly becuase the stock carb worked best off road, at least for me(Rubicon trail, other Sierra rock trails).
This is a great website; lots of good knowledge here!
My best guess would be it's a 625cfm, but I'm not certain. Thats a bit big for a 225-6cyl. It's definitely an early carb judging by the cable-style kickdown lever on the throttle linkage.