These rules are slowly becoming their own thing but they are nevertheless heavily indebted to the following games:
Both of these are ‘retroclones’ of the original Dungeons & Dragons rules from 1974 (also known as 0 Edition, or 0e for short), although both contain a few interesting interpretations of the original rules. ‘7VoZ’ in particular diverges quite siginificantly in many ways – almost entirely for the better, so far as I’m concerned – and even Delving Deeper integrates a few concepts from ‘Chainmail’, a miniatures wargame that was used as the earliest combat system in the proto-D&D games played by Dave Arneson and Gary Gygax, so it is also a creative work in it’s own right.
In cases where Delving Deeper and 7VoZ have conflicted, I have generally deferred to the 0e rules as per Messrs. Gygax and Arneson.
I have also referred to what is called the ‘Holmes Basic Set’, actually an edited edition of the 1974 rules and their first supplement, ‘Greyhawk’, albeit with rules only for the first three character levels and intended as an introduction to ‘Advanced Dungeons & Dragons’ – although that wasn’t published in full until 1979, two years after the Holmes set, when the first Dungeon Master’s Guide was released.
J. Eric Holmes was an early player who felt (correctly) that 0e was unnecessarily vague, so he produced a new version of the rules to smooth over the rough spots. While he used the original wording wherever possible, it nevertheless includes some interesting additions to the original 0e rules where they were lacking – in some cases quite different to what Gygax himself ultimately produced (see below!).
Occasionally I’ve also referred to Advanced Dungeons & Dragons (first edition, or 1e), the final version of the D&D rules produced by Gygax himself. That said I have generally avoided doing so as AD&D was released in dribs and drabs from 1977 to 1979 (starting with the Monster Manual), ie. it is hopelessly newfangled in most ways and therefore unsatisfactory to a recalcitrant reactionary such as myself.
I borrowed the ‘single saving throw’ concept from Swords & Wizardry, which is a competing (although not as cool compared to Delving Deeper) retroclone of the 0e rules, that includes a lot of stuff from the original supplements. There is a free PDF version.
If you are interested more generally in the ‘OSR’, as in the late 2000s movement to justify, popularise and play the early editions of D&D (of which my work here is self-consciously an offshoot), then I strongly recommend reading this series of posts by ‘OSR Simulacrum’ on Blogspot (I’ve provided archive.is links for additional future-proofing):
Parts I-III cover the history of TSR and D&D (essential for understanding all this business), while Part IV covers the OSR itself. Part V is especially helpful, as a taxonomy of the various modern OSR substrains.
I think my work here qualifies as what he’d call a ‘D&D Mod’; I’m not particularly interested in ‘modernising’/debasing the game, just consolidating the best parts of the old versions and the more conservative clones. Even my ‘innovations’ (‘race as multi-class’, the weapons vs. armour stuff) are still rooted in the original rules.