This single piece of paper constitutes the entirety of the understanding between TSR and the authors of the 1975 Greyhawk supplement: most significantly, it covers a copyright assignment to TSR and an agreement from TSR to pay royalties based on "the cover price of the game rules or game on each and every copy sold." It is quite short, having been drafted pro bono by Gary Gygax's uncle at a time when Tactical Studies Rules was still a partnership, and D&D had sold perhaps 1500 copies. This language is of especial interest because the same form was used for the 1975 D&D contract, and famously Dave Arneson would later sue TSR on the grounds that his rights to royalties extended to titles like the Holmes Basic Set and the AD&D hardcover books. This is a major focus of Game Wizards.
Over the next five years, TSR refined the language of their new agreements to be clearer about what rights authors were assigning, and for which sales they would receive compensation. I am definitely not a lawyer, but I am going to offer below a few layman's thoughts about the evolution of TSR's contract language during this period.