This topic just came up again on Dragonsfoot, and reminded of a post I made on OD&D Discussion early last year before I started this blog, which I've just revised:
The Holmes Basic set was first published in mid-1977. The AD&D Monster Manual first appeared after Xmas in late Dec 1977. The rules for AD&D were still in development at the time, and the Monster Manual refers to a number of rules from OD&D or Holmes that were later changed by the AD&D Player's Handbook (mid '78) or Dungeon Master's Guide ('79).
- The entry for Orcus in the
Monster Manual indicates that his "tail strikes with an 18 dexterity
which does 2-8 hit points each time it hits" (pg 17). To me this implies
an attack compatible with the Holmes rules for initiative where first
strike is determined by dexterity. There is one other monster in the
Monster Manual that has a dexterity score in its description: Brownies
have "18 dexterity" (pg 11).
- Yeenoghu can cast "magic missile (3/day,
6/missiles/cast), each doing 2-8 points of damage and having a +2 to
hit" (pg 20). In AD&D magic missiles strike unerringly, but Holmes
Basic requires a "to hit" role - "Roll the missile fire like a long bow
arrow" (pg 15) - so Yeenoghu's bonus to hit with a magic missile refers
to this rule. This was discussed in the comments on a Grognardia post.
This may indicate that Gary intended the OD&D magic missile to
require a "to hit" roll. The original description of Magic Missile in
the Greyhawk supplement is entirely unclear as to this, as it just
describes the magic missile as "equivalent to a magic arrow". Tim Kask
has stated on DF that the OD&D version did require a roll, and was purposefully changed for AD&D.
- The Monster Manual basically has a five point alignment
system like in the Holmes Basic rulebook, which in turn was derived from
an article in Strategic Review #6 (Feb 1976). A few monsters in Holmes
do not conform: the Displacer Beast is "neutral (evil)", and
others have two alignments (e.g. Dopplegangers are "chaotic
evil/neutral"). Likewise, the Monster Manual has a few monsters with
"neutral (evil)" or "neutral (good)". But neither has the full-fledged
nine-point alignment system that later surfaced in the Players Handbook.
- As described by T. Foster here, no monster in the Monster Manual has an AC 10. I note that new "armorless" monsters like the Gas Spore and Nymph are AC 9, and previously described monsters like the Gelatinous Cube, Gray Ooze, and Ochre Jelly are AC 8, just like in both OD&D and then Holmes Basic.
- T. Foster lists other details compatible with OD&D: "spell-names that were changed in AD&D (e.g. raise dead fully vs.
resurrection), the max. levels listed for demi-human leader-types match
the level limits in OD&D (AD&D's are usually 1-2 levels
higher), and ... casting-level equivalents that
match the OD&D rather than AD&D tables". The last is further described by papers&paychecks in this thread, where he points to Nagas as a good example of the "casting-level equivalents that match OD&D". For example, the Spirit Naga has "4th level clerical ability" which is "2 - first and 1- second level spells per day" (pg 73). This is consistent with OD&D, as well as the implied progression in Holmes, where 3rd level clerics get two 1st level spells.
My conclusion is that at the time the Monster Manual came out it was most compatible with the Holmes interpretation of the original D&D rules. But what this really shows is that some AD&D specific rules were developed after Gary finished working on the Monster Manual. Let me know if you notice any other instances that align with Holmes or OD&D.