Autarch recently put a very old draft up on the Patreon, the first version of ACKS, back when it was just a bunch of houserules for B/X. It's almost like they heard I was looking for an intermediate / transitional ruleset!
Things that I found interesting:
Alex's erudition and attention to citations never ceases to surprise. He cites a number of old D&D books that I had never heard of, including The Golden Khan and Dwarves of Rockhome, and a few I have heard of but never read, like Orcs of Thar and Glantri. There's also a reference to "The Dominions at War rules supplement", which google only finds two results for; one in a forum thread at Autarch, and this post. So I assume that that was a free early draft of Domains at War. It would be pretty interesting to see how those rules evolved too.
He also cites the Ready Ref sheets in a couple of places. I had them in pdf but had never really figured out which parts of them were worth using, so it was nice of him to pick out some of the good bits. In particular, the Rudimentary Resurrections table is apparently the grandfather of ACKS' Death and Dismemberment and Tampering with Mortality tables. The Proclamations and Town Crier tables were also interesting.
Max HP at first level.
A lot of stuff that I think of as being uniquely ACKS was already in place.
any single adventure, magic-users and clerics can cast the number of
spells of each level from among their spells known as indicated by
the Spells & Level chart" (emphasis mine)
Proficiencies used to be called Feats.
Slower fighter damage bonus and thief backstab multiplier progressions, which is something I've kicked around myself.
The trade goods table is apparently not derived from any other source; this one has the encumbrance per load in coins! He's also already worked out the demand modifiers for a bunch of cities in the Auran setting (and apparently market classes used to be A-F instead of I-VI... an inheritance from Traveller starport classes, perhaps?).
The magic research, constructs, magic item creation, necromancy, etc rules are already out in full force, including lists of monster components for particular items. I didn't read them in enough detail to compare them with the final rules.
Custom spell lists per cleric patron deity; I think a lot of these got rolled into ACKS' cleric list by default, which is part of why the spell list is bigger than B/X's.
Some classes that didn't make it - Charlatan (thief / AD&D illusionist cross), Elven Druid (both arcane and divine casting), Mariner (nautical explorer variant). Craftpriest started as Craftmaster (with a bunch of skills and "general feats") and only picked up divine casting later. They still had infravision.
Many skills were still on 1d6.
So all in all that was a very interesting and well-timed piece of archaeological material, and I hope we'll see more of this sort of thing on the Autarch patreon.