Additions and Subtractions Over Time; Whence the OD&D Sup-I Spells?
While I personally got into D&D initially with the 1979 Holmes Basic D&D Set, I had a somewhat more well-resourced friend who later got the 1981 Moldvay Basic/Cook Expert Rules sets, and actually handed them off to me, so as to DM him and other friends through higher levels of play. (I was always the DM in our community since day zero, so this just seemed like the thing to do.) We played a lot with those rules for a bunch of years, and every time I go back to them I'm really impressed by the level of reflection and tasteful editing that Moldvay & Cook put into those rules. While Holmes was very connected-at-the-hip to AD&D (with the Holmes text being edited & altered by Gygax himself before printing), the Moldvay-Cook B/X intentionally set its own traditions (like race-as-class), a separately-evolving game through the 1980's. Let's take a comprehensive look at their magic-user spell lists here.
Moldvay Basic Rules
One thing that Moldvay does here is to set a tradition of exactly 12 spells in each level of magic-user spells. That's a little bit nice, since you know you can always roll a d12 for a random spell in any level (as a scroll, spellbook, or maybe an NPC wizard's memory); although it may be a bit of a handcuff in the design space. Recall that the OD&D had no such organizing principle, different levels had different numbers (and in distinction to AD&D, there were actually more at higher levels, not less).
In the LBB's, at there were only 8 spells at 1st level, and 10 at 2nd level. Therefore, to fill out his list, Moldvay had to include some of the canonical D&D spells which first appeared in D&D Supplement-I, Greyhawk -- at 1st level, magic missile, shield, and ventriloquism; at 2nd level, mirror image and web. Now, at first level, that still doesn't fill out his d12 roster, so Moldvay also had to go to AD&D and take the floating disc spell (a.k.a. Tenser's floating disc in those rules). Note that this is the only spell in the entire B/X line that didn't appear in either the LBBs or Sup-I. On the other hand, there was an overflow of OD&D Sup-I spells that didn't make it into Moldvay at second level: namely darkness, strength, magic mouth, and pyrotechnics (several of these being pretty canonical for D&D -- I recently saw fellow players quite surprised that strength and darkness were not available playing by these rules).
Now before I go on, let's compare this to the Holmes Basic D&D list. Holmes' original manuscript had lists that were identical to Sup-I (although alphabetized and unnumbered -- 11 spells at 1st level, 16 at 2nd level). However, Gygax got in after him and added several spells that otherwise only appear in AD&D -- dancing lights, enlargement, Tenser's floating disc (by its full name), audible glamer, and ray of enfeeblement. This then brought the total number in the published Holmes work to 14 at 1st level, 18 at 2nd level (more than in the later Moldvay rules). If you want to see a complete look at that development from Holmes' unpublished manuscript, see the recent Zenopus Archives Blog (spell levels one, two).
Cook Expert Rules
In Expert D&D Rules, Cook starts by repeating the Moldvay list, and then adds the spells you see above for levels 3-6. (Asterisks represent reversible spells, also added as appropriate to the 1-2 level lists). He continues with the design pattern of exactly 12 spells at each level. But here's a wrinkle: since the OD&D LBB's already had that many spells or more at each of these levels (14 at 3rd, 12 at 4th, 14 at 5th, 12 at 6th), he didn't need to copy any Sup-I spells into the Expert rules, and in fact even a number of spells in the original LBB's got cut out (clairaudience, slow at 3rd level; growth of animals, wall of iron at 5th level). All the higher-level Sup-I spells went entirely missing from these rules; and that's probably for the best, because they tend to be wonky, confusing, or under-powered anyway.
Later EditionsThe soon-revised 1983 Frank Mentzer Basic/Expert Rules keep almost the same lists as Moldvay-Cook -- except that Mentzer cuts the spells at 5th & 6th levels down to just 8 (other levels have the same 12 as in B/X); maybe for space purposes in the book?
On the other hand,the 1991 Aaron Allston D&D Rules Cyclopedia makes the very strange choice of adding one spell per level to B/X, and thereby having a uniform 13 spells at each level (and thus not immediately rollable on any Platonic die). The spells he adds are, respectively by level 1-6: analyze, entangle, create air, clothform, dissolve, and stoneform. (He also has level 7-9 spells based on the Mentzer Companion/Master Sets, which I won't go into here, but likewise have 13 spells in each list).
Was there perhaps some intermediary product in the late 80's that add these "thirteenth" spells to Basic D&D along the way?