Thanks to Allan Grohe (grodog), I had the opportunity to exchange a few emails with J. Eric Holmes' son Chris last year. One thing that Chris mentioned to both of us was that he and his father learned to play D&D from a group of high schoolers who used the Warlock D&D supplement published in Spartan #9 (Aug 1975). They only played once with that group but Chris implied they continued using those rules, only starting a new campaign with regular D&D rules when his father was editing the Basic rulebook. As I mentioned previously, Holmes later wrote in his FRPG book (1981) that he used the combat table from Warlock when he was starting out. So for a few years (~1975-76), Holmes was using the Warlock supplement, which alters/clarifies/expands the Men & Magic portion of OD&D. Here are a few other places where Holmes shows the influence of the Warlock rules:
● Prior to editing the Basic Set, Holmes contributed several campaign stories to the APAzine Alarums & Excursions, some with Boinger & Zereth of the later Dragon stories. In the "Adventure of the Giant Chameleon" (A&E #14, August 1976), the characters find scrolls with Awaken and Charm Mollusc spells (Update: a wizard also casts Charm Reptile). Warlock has an Awaken spell, a second level M-U spell that cancels sleep spells or drugs. Warlock doesn't have a Charm Mollusc per se but the first level M-U spell list does have a Charm Amphibian, Charm Arachnid, Charm Avian, Charm Crustacean, Charm Fish, Charm Insect, Charm Mammal and Charm Reptile - a diversity that likely inspired Holmes' Charm Mollusc. The story also makes reference to a Charm Reptile spell. These references show that Holmes was using some of the Warlock spells during these early years.
● In Dragon #52, Holmes mentions he tried to convince Gygax to include a spell point system in the Basic Set. Warlock has an extensive spell point system where each spell is given a separate point value. So Holmes may have been using this spell point system.
● While neither of the above made it into the Basic rulebook, part of the combat system did. Warlock has an "order of combat" not specified in OD&D or the Greyhawk supplement (movement, magic, archery and then melee):
"Each game turn [one minute] consists of six phases of ten seconds each. Each phase is broken down into four parts: Movement, Magic, Archery, Melee; It is not necessary to operate in phases except when combat occurs, for the simple reason that is is not needed when simply walking around. The parts take place in the order stated during any individual phase. First, all characters in the situation move to any new positions. Then any magical spells are thrown, in order of dexterity of the magic user from high to low. (This step includes any implements, such as rings, used by other characters as well.) Next comes archery, during which dragon breath and other projectiles are hulled. Finally comes melee, with hand to hand combat being resolved" (pg 5).
"When there is time, or when a magic-user says he is getting a spell ready, magic spells go off first. This is followed by any missile fire, if the distance to the monsters permits, and then melee is joined, after which no missile fire is permitted because of the danger of hitting friendly forces" (pg 21).
While Holmes doesn't mention movement here, the rest of the order is the same: magic, missile fire, melee. Presumably Holmes used what he was familiar with from Warlock to help clarify an aspect of combat for Basic.
Other possible influences on Holmes Basic combat found in this same paragraph of Warlock are the length of combat rounds (10 sec) and the use of dexterity, though with these sorting out influences is complex because the Metamorphosis Alpha RPG also uses both of these in the year (1976) before the Holmes Basic Set. Furthermore, the concept of using dexterity to determine missile/spell order can be traced back to the original D&D rules: "Dexterity ... will indicate the character's missile ability and speed with actions such as firing first, getting off a spell, etc" (pg 11, Men & Magic). Also, Warlock only mentions dexterity for determining spell order (a concept seemingly derived from the OD&D rule) but not melee combat as in Metamorphosis Alpha and Holmes Basic.