The problem is that this particular version of M&T doesn't match any edition of the D&D rules. There was an earlier edition (1977-78, in three separate booklets), which presumably matched the OD&D rules at that time. But a revised edition was released later (1980, collected in one book), with widespread editorial changes (one might say "mangling") throughout, and that's the version available in PDF form. The Acaeum has a nice page listing all the changes between the editions, compiled by Trent Smith, wherein he asserts the following ( http://www.acaeum.com/library/m&tchanges.html ):
Obviously a lot of these changes, especially at the lower levels, were made to try and match the content and conventions of the Moldvay-edit Basic Set (type-names for snakes, lizards, spiders, etc., class level titles, no paladins, etc.), but it's equally obvious that whoever did the editing/updating had no real idea of what was going to be in the Expert Set.
Here's some of the oddities that Trent doesn't mention. The stat blocks contain hit points, but no Hit Dice. That said, the hit points are calculated assuming 8-sided hit dice (best seen in the Hydra entries), which would distinguish it from Original D&D. However, there are no damage entries, assuming that all base damage is 1d6 (which was changed in OD&D at the same time that HD became d8). And yet, there are things like multiclassed Halfling Hero/ Thieves, which don't exist in the Holmes or Moldvay Basic sets. And finally, there's a statistic called "AL/Attack Level", which was never seen before or since in any D&D product of any sort to my knowledge (not the same thing as an "attack bonus", it starts at 10 and goes down with greater ability: possibly roll d20-AL+AC >= 10 for a hit or something).
In summary: The 1980 M&T Assortment isn't strictly compatible with any published D&D ruleset, ever.
Question for anyone who has the earlier printing (1977-78): Did it originally use d6 Hit Dice? (Most easily checked in the Hydra entries: heads x dice = hp.) Was it at least originally compatible with OD&D?