Empire of the Petal Throne

A major variant from TSR
Released in 1975, one year after the LBBs, Empire of the Petal Throne (EPT) is what I call a variant on the original rules. EPT makes some major changes to the original rules, but not so much as to make them unrecognizable.  Changes include use of percentile dice for rolling up characters, substituting Psychic ability for Wisdom as one of the attributes/talents, new spell system for magic users and clerics (including success rolls), a percentile based skill system and alignment based on good and evil rather than law and chaos. EPT also introduces the setting of Tekumel and the rules are closely adapted to gaming in that unique milieu.
Many aspects of white box play remain the same, however. The three original character classes of the LBBs renamed "professions", the combat system, saving throws, treasure and the leveling up concept remain virtually unchanged except for some terminology.
Since the publication of EPT there has been much more written about the world of Tekumel, including several novels by the creator Prof. M.A.R. Barker, but what is included in the original TSR box set is enough to get the referee running adventures in Tekumel. The spiral bound illustrated rulebook includes information on background history, geography and climate, religion and deities, about a dozen intelligent races, their languages and around 70 monsters. There are two colorful world maps and a city map included in the box. These are plastic maps rather than paper and I assume accounted for a good deal of the $25 original price of the game.
The popularity of Tekumel as a setting has generated a large body of information on that world which wasn't available in 1975, at least to anyone other than Prof. Barker. Taking Prof. Barker's advice to the referee to read what's in the box and then develop your own version of Tekumel through play is exactly what I think of when I imagine what the original game is all about. (Don't be afraid to make it your own!) In his preface to the game, Mr. Gygax writes that EPT is something special in the context of gaming, a world created with roleplaying adventure in mind. I see EPT as a powerful example of the flexibility of the white box rules Mr. Gygax and Mr. Arneson created and of their reinterpretation by a first class world-builder, Prof. Barker. Together these pioneers created EPT and demonstrate to the rest of the gaming hobby what is possible when using our imagination and building upon the work of others.