USMC camo. from WWII is the fore runner of Duck Hunter commercial print which was used by the CIDG and US Advisors early in VN conflict (around '62 or earlier). The Marine pattern as seen in gi.rene's photo is aka. "Frog-Skin", whereas "Frog & Leaf" refers to ERDL pattern developed in time for mid-VN conflict issue. The nick name supposedly was given by members of US Army L.R.R.P. (Lurps) who may have been issued the ERDL with priority. USCM camo. was a sub set of the "Frog & Leaf" in that it did not include the entire ERDL repeat but only a part (roughly 1/4 of the original ERDL print). The neat part is that someone quite clever made the sub set also repeat. By "repeat" I mean that the camouflage pattern repeats itself every so many inches. In USCM camo. the repeat occurs roughly every 36 inches, whereas the ERDL repeat might be substantially more lengthy (don't know exactly). I think I also saw evidence that the ERDL pattern is printed in a sideways direction to the USCM camo and vice versa, which is really mind tripping (for camo. print nerds like me). Hope that helps. Back on-topic.
More Geek Talk: If you look at USMC WWII "Frog-Skin" -- the pattern does reminds of smaller tree leafs. But it also look somewhat like animal patterns, e.g. certain cat species like leopards for instance. If you look at ERDL "Frog & Leaf" there are shapes that do not appear in the former. These shapes reminds more of twigs and branches (the dark colored ones), as well as other shapes one might expect looking at foilage and inter-woven shadowing in the jungle.