From the publisher's website:
"The Labyrinth Lord fantasy role-playing game allows you to experience the game play of the retro-editions of the world’s most popular fantasy role-playing game! The Labyrinth Lord core rules emulate the rules and game play experience of the 1981 edition of the world’s most popular fantasy role-playing game. The 1981 version reorganized and clarified the rules from the very first version of the game released in 1974, so it is the best version to pick up and play with little prep time...
"The Advanced Edition Companion is a player’s handbook and referee guide that allows the use of the array of advanced character possibilities from the 1978 (first edition) rules, and first edition monsters and magical items. No more pining away for all the great classes and races! All character class options from the Labyrinth Lord core rules, the Original Edition Characters book, and the Advanced Edition Companion book are cross-compatible. No player has to sacrifice his/her tastes for one game, since all classes from any version can be played together."
- Advanced Edition Companion