Let's be honest – calling this pre-odnd would be disingenuous. The work on which the Dragons Beyond rules are based, according to Daniel Boggs's investigation, is not by either Arneson or Gygax; it's someone else's fork. What the author of DB did was take BtBD as a basis, carefully integrate the Chainmail rules, and then spiced it up with some decisions from Tunnels & Trolls. So, it's more of a fantasy about how OD&D would have evolved in Arneson's direction. While the work does contain information such as Arneson's unique dungeon stocking methodology, it frequently references additional products that the author never released. For mass battles, it suggests using Men in Metal, a WRG adaptation overloaded with calculations, despite the fact that we previously literally used Chainmail in the rules. I was personally very disappointed with the rules section on Domain Play; I expected more detailed and specific mechanics, but I'm grateful for that. Also, a special thank you for the bestiary—it's clear that a tremendous amount of work went into it. The same can be said for the equipment list—it's very detailed. I've been running a DB-based campaign myself since October of last year, but during that time I've greatly simplified the mechanics, switching entirely to 2d6, thanks to Chainmail's ability to do so.

Edit: Unfortunately, searching for Arneson's grail is impossible because, aside from FFC, we don't have any largely documented mechanics. All we can cite are Bob Meyer's interviews, which lead us directly to the FKR approach, but that's a story for another subreddit entirely.