Source of the 4100 Autolite 1.12 "600cfm" quote???

I've seen the 4100 Autolite 1.12 quoted as a "600cfm" carb in numerous places, and yet the Ford literature I know of says
1.08 = 440cfm
1.12 = 480cfm

Over on the FE Forum, Bob Sprowl had a bunch of 1.12 carbs measured, they averaged 520cfm, all were between 505-528cfm, but 600cfm, no way.

Edit: You can read about Bob's tests over on
fordfe.info
The 1.12 4100's averaged 520cfm over four carbs.
The 1.08 4100 gave 440cfm, Bob only tested one.

Is the "600cfm" a measured number for the stock Autolite 1.12, or what is its source? It seems (too) big a number-