As well as the link above, there is also a wiki on them under the name 335 series engines (dubbed as Cleveland).
http://en.wikipedia.org/wiki/Ford_335_engine
So it appears,
Valve sizes
Port sizes
Open v Closed chamber
Compression
Carb which is where the term 'V' officially comes from?
From the wiki it appears that in the US at least, all 4 barrels were
4V, and all 2 barrels were 2v which is why the association is carb based, but given there appears to be differences in port sizes and chamber sizes that will explain how some say its carb based, and others argue it isnt as there are other differences than simply carb and inlet manifold. But Im not sure how accurate the wiki is, as Im pretty sure there are both open and closed chamber
4V heads. This Im less certain of, but I THINK there are also open and closed chamber 2V heads.
Im sure someone here will have the ins and outs and clear it up.