"At the end of each three turns the Dungeon Master can roll a die to see if a wandering monster has come down the corridor. A roll of 6 means that something has come "strolling" along" - Holmes Basic rulebookEric Harshbarger, the Lego sculptor and game/puzzle designer who built a Lego mosaic of the Sutherland Dragon, has made Wandering Monster d12s based on the Holmes Basic encounter tables (3rd edition version). The post on his site is partially quoted below:
This is probably the most specialized set of dice I've ever made, and it will appeal only to those who are as enamored with "old school" role-playing games as I am. The original Basic Dungeons & Dragons game had tables in it for "random encounters" or "wandering monsters". If something was encountered by an adventuring party, the dungeon master was supposed to roll a twelve sided die and refer to one of three tables (depending upon what "level" of the dungeon the party was on -- the rulebook only considered dungeons of depth down to level three).Click here to read the rest of his post.
Click here to read about changes to the Level 1-3 Wandering Monster Tables from OD&D Vol III to Greyhawk to the three printings of Holmes Basic.