Over on the Piazza, a great forum dedicated to the various and sundry D&D settings, member TraverseTravis has posted scans of a sheaf of documents from the Original Known World D&D campaign of Lawrence Schick and the late Tom Moldvay before they started working for TSR. This is the original material which Moldvay and others drew on for developing the published Known World campaign setting - later Mystara - as it initially appears in the 1981 D&D Expert Rulebook and the module X1 Isle of Dread.
Portions of this material surfaced back in 2015, which I described in post on this blog called "Ur-Known World"; from there you can find a link to an interview with Schick on Black Gate.
The new set of documents was saved by one of the original members of the group, Bill Wilkerson (thanked as a playtester in Moldvay's module B4 The Lost City), who provided them to TraverseTravis for scanning. Shannon Appecline compiled the scans into a single document organized by subject according to OD&D Vol I-III.
Find links for downloading them in the first post here:
Bill Wilkerson's Original Known World documents
I've been contributing to the Piazza thread with comments on how the "New Monsters" from the Original Known World were later used by Moldvay and Schick in various TSR publications such as the Fiend Folio, X1 Isle of Dread and X2 Castle Amber.
A big thanks to TraverseTravis and Shannon Appelcline for their efforts in making this material available to the public!