The Holmes Companion is an unofficial fan-made expansion that extends the Holmes Basic D&D rules to allow play from levels 4 to 9. The game as released was viewed as an introduction to AD&D and only had rules allowing play to level 3.
The Holmes Companion
A Level 4-9 Rules Expansion for TSR’s 1977 D&D Basic Set
The Original 1974 Dungeons & Dragons game, as well as discussions that took place on Dragonsfoot, the K&K Alehouse, Finarvyn¡ s OD&D Forum, and especially Philotomy's OD&D Musings inspired this four-page rules expansion. The goal was to make "Holmes D&D" a gritty, more focused, and complete name-level (9th) dungeon-crawling game without requiring the Advanced Dungeons & Dragons rulebooks for expansion, thus keeping it more faithful to the mechanics and feel of Original D&D.