|The Jim Lamb DMG. Source: Ebay Auction|
Above is a screenshot from a recent auction of a 1st edition Dungeon Masters Guide, inscribed by Gary Gygax with: "2 August 1979 / This is the 1st copy of the DMG other than my own! for Jim Lamb Gatherer of the Mighty Brador! Gary Gygax"
Searching for the two names together, I found a column by Gygax on page 11 of Dragon #27, July 1979, titled "CANGAMES ’79 A brief report on the Canadian Gaming Convention at Ottawa, May 18-21, 1979", which relates that "Jim Lamb, one of the Cangames organizers, telephoned the motel, and after we had supper nearby, took me over to the convention site on the campus of the University of Ottawa". Possibly Gygax sent this copy to Lamb in return for his hospitality at the convention. Per the Acaeum, the DMG was first available to the public at Gen Con XII, August 16-19, 1979.
This putative "second copy" of the DMG was recently auctioned by Jim Lamb himself, and was bought by someone for an astounding $4500!