You got that at a thrift store?! And for $20?!?! That bro, is the original red box. That is worth its weight in gold. You're looking at one of the earliest rules-lite versions of AD&D and it dates back to 1983! Treasure it bro.
Quote from Wikipedia on it
In 1983, the Basic Set was revised again, this time by Frank Mentzer, and redubbed Dungeons & Dragons Set 1: Basic Rules. The set included a sixty-four page Players Manual, a forty-eight page Dungeon Masters Rulebook, six dice, and in sets in which the dice were not painted, a crayon. The 1983 revision was packaged in a distinctive red box, and featured cover art by Larry Elmore. Between 1983 and 1985, the system was revised and expanded by Mentzer as a series of five boxed sets, including the Basic Rules (red cover), Expert Rules (blue), Companion Rules (teal, supporting levels fifteen through twenty-five), Master Rules (black, supporting levels twenty-six through thirty-six), and Immortal Rules (gold, supporting Immortals, characters who had transcended levels). Instead of an adventure module, the Basic Set rulebooks included a solo adventure and an introductory scenario to be run by the Dungeon Master.
The 10th Anniversary Dungeons & Dragons Collector's Set boxed set, published by TSR in 1984, included the rulebooks from the Basic, Expert, and Companion sets; modules AC2, AC3, B1, B2, and M1 Blizzard Pass; Player Character Record Sheets; and dice. This set was limited to a thousand copies, and was sold by mail and at GenCon 17.
An Australian version of the Basic Set was printed by Jedko Games in 1987.