While researching Playing at the World, I spent years looking for a playtesting edition of the original Dungeons & Dragons. Early sources suggested that some of kind preliminary draft was circulated; as Gygax in 1977 remembered, "we began serious play-testing in Lake Geneva, while copies were sent to the Twin Cities and to several other groups for comment." If such a draft was indeed so widely distributed, the failure of any copy to survive seemed hard to explain. The problem, however, was that we didn't even know what we were looking for. There were only vague references to a title, like "The Fantasy Game," but no one turned up anything meeting that description. It is therefore not surprising that a document like the Dalluhn Manuscript could, for so long, hide in plain sight. It seems to be nothing less than one of these long-sought pre-publication editions of OD&D.
[EDIT: I've left this post up to reflect the beginning of my research; for my complete analysis of this document, see this post here and further confirmation based on the discovery of a 1973 Gygax working draft here.]