I have several early afb's without an air flap. Like 66fs's, the boosters are from the side. The secondary barrels are small enough that I reckon the air flap wasn't necessary. Don't know the cfm's, but I'd bet they are all well less than 500, maybe less than 400.
The one on my 63 Valiant is a 3249S from a 1962 poly 318 automatic (I think last year for power pack poly 318).
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Also have a 2991S from a 1960 318 poly and a 2856S from a 1959 361.
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By the way, 66, your 3108S shows as being from a Chrysler Marine 413
