When I compiled Supplement V: CARCOSA, I based it solely on the 1974 D&D rules. This was, in my mind, only the second-best possibility.
What would have been even better is if I had access to the rules that Dave Arneson used before he gave a copy to Gary in the early 1970s. What he have Gary was 18 hand-written pages. If I had had a clean copy of that, CARCOSA would have used those rules as its base.
It would be awsome if they would come to light someday, but I suspect Gygax didn't preserve them (He later claimed they were "almost useless") and Arneson didn't copy them. I think the majority of Arneson's existing notes made it into FFC and after the legal disputes he seemed reluctant to give out those kind of details. At this point though, one would think those records could be unsealed.
I wonder if there are clues to the 18 pages in the old C&C Society newsletters. Sadly, those are such collectables and the folks who own copies are unwilling to share (I believe) because they are afraid the price will fall. I'm pretty sure that there were some proto-OD&D articles from the days when Chainmail was evolving into OD&D and some early pre-OD&D Blackmoor articles as well.
I suspect that Adventures in Fantasy is far more complex, as it fills up 3 volumes with rules, but that many of the ideas in First Fantasy Campaign were closer to the original (except that Dave liked using percentile dice a lot...).
Marv / Finarvyn DCC playtester (2011) S&W WhiteBox author (2009) C&C playtester (2003) Builder of the TrollBridge for T&T; Amber Diceless player since 1993 OD&D Player since 1975; Metamorphosis Alpha since 1976
"Don't ask me what you need to hit. Just roll the die and I will let you know!" - Dave Arneson
Arneson said in one of his interviews (in Pegasus I think) that he argued for a spell point system in OD&D instead of Gygaxs Vancian system, so that was probably in those 18 pages, and he said the magic system in AIF was "pretty much" they way he ran it in blackmoor. I've collected some notes of things from AIF that seem to have Blackmoor roots but the magic system is probably the biggest thing. I'd love to see those C & C newsletters. Knowledge should be shared.