To recap the gist of the previous post, combat rounds are aggregate abstractions of all the action in an interval of time (in B/X, 10 seconds.) Choices in combat are not discrete maneuvers but broad tactical decisions applied to the entire round. The attack roll represents not a single attempt at attacking, but the likelihood of a combatant seeing and capitalizing on at least one opportunity to strike or rattle the target. A miss doesn't necessarily mean an earnest swing that fails to connect; it could also mean that no good opportunity was presented at all. Damage isn't necessarily done only by the attacker's weapon; the weapon merely determines the range of the attacker's damage potential.
Now that that's out of the way, I want to explore some special cases, and a few instances in the B/X rules that seem to ignore the principles behind the abstract aggregation of the ten-second round.
First, there's initiative. The most intuitive interpretation is that initiative literally determines who strikes first. Since we don't even know for sure until we roll attacks that an opportunity to attack has even occurred, we can't even say that initiative represents the first opportunity. I like to think of it more as an abstract measure of the ebb and flow of combat, a momentary edge which could permit a combatant to take his opponent out of the fight IF he can spot and capitalize on an opportunity. It's worth noting, though, that the game would actually function just fine without initiative at all - all combat takes place simultaneously. This might slightly increase the lethality of the game, because double kills would become more common, but the possibility of initiative-less combat intrigues me nonetheless.
Missile combat is troublesome because unlike melee combat, it uses up resources; specifically, ammunition. Even so, the number of arrows or bolts or sling stones expended doesn't necessarily correlate 1:1 with attack rolls. You can hand-wave it and say that the number of missiles expended averages out to one per round (sometimes you get off a couple shots, and sometimes you don't get a clear look at all.) You could also use an abstract method of tracking ammo.
Next, there's the issue of multiple attacks. B/X as written doesn't support multiple attacks for characters (at least not without magic like the haste spell) but BECMI and other old school editions do. It's not too difficult to grasp that a more skilled fighter will, on average, see and be able to take advantage of more opportunities to strike in a given time than a less skilled one. But, since we can't definitively say how many opportunities a character exploits with a successful attack roll, it's a little difficult to fathom what a second or third attack roll by the same character in the same round might mean. That they can potentially exploit twice or three times an indeterminate number of opportunities?
It seems to me that this might be more aptly modeled, and more consistent with the combat round as I understand it, by increasing damage potential rather than number of attack rolls. As characters increase in level, their chance to exploit at least one opportunity in a round increases with their improving attack rolls. Since the damage range given for a weapon doesn't necessarily represent the damage caused by one stroke of that weapon, it seems reasonable that damage per round could be increased for highly skilled characters without absurdly implying that they're necessarily doing a lot more with a single blow. You could increase the damage die size a step or two, for example 1d6 to 1d8 or 1d0, or grant extra dice of the same type as the weapon's base damage, say, from 1d6 to 2d6. The latter pretty neatly captures the spirit of multiple attacks, without messing with the base damage die for each weapon.
But wait a second...What if multiple attacks don't necessarily mean multiple swings (since we know that happens anyway) but the ability to threaten additional opponents in the same round? Say, a fighter with one attack per round, surrounded by orcs, could only attack and potentially do damage to one of them in a round, but a fighter who gets two or three attacks per round could attack two or three of them, and divide the damage rolled among them? This could still be handled with a single attack roll. If the targets have different ACs, the same roll is compared to all; some may take damage and some not. (Whether the damage should be divided by the total number of targets, or only between those that are actually "hit" is a question for which I don't have an answer yet.)
And now we come to the weirdness of monsters. Unlike characters, the multiple attacks of monsters are normally listed with specific attack forms for each, implying that, for instance, a grizzly bear makes one swipe with each paw and one attempt to bite in a combat round. That's a pretty neat and orderly attack routine for a wild beast. It would make more sense to aggregate the possible damage and use a single attack roll. A B/X grizzly does 1d4/1d4/1d8, so in the single attack format, 2d8 would be about right. (Rather than the bonus "hug" damage being activated on two claw hits, it could occur on a critical of 19 or 20 of the single attack roll.) It's not unrealistic to suppose that a bear could maul three closely-grouped opponents at once, so multiple attacks could be directed against two or three characters, and the damage rolled divided among them. (Naturally, only one, probably chosen at random, could be subjected to the "hug.") Now we need not concern ourselves about whether the bear went after Bob the fighter with a claw or a bite attack. We just know that the grizzly tore into Bob for 8 points of damage, and whether it was its slashing paws or snapping jaws or some combination that got him can be narrated any way the DM and player feel appropriate.
Some creatures, with huge damage potential and equally huge bodies, like dragons, may be able to attack even more than three targets at once. A big amorphous blob like a black pudding might be able to threaten everyone within striking distance of it at once.
This does make for a more all-or-nothing experience in combat; a creature either does its full damage or none. This would be an advantage for well-armored characters and a disadvantage for lightly or unarmored ones. A monster which gets three attacks under the rules-as-written would do at least some damage 48.8% of the time against a target that it needs a 17 to hit; with a single attack roll it has a 20% chance to do its full damage. The same monster, against a target that it needs a 6 to hit, will hit all three attacks 42% of the time; with the single roll it will do its full damage 75% of the time. I don't have a problem with this, but your mileage may vary.