I think all of us who have seen this picture of Gary Gygax running Greyhawk have spent a few minutes speculating about it.
Since Evan at In Places Deep asked about the map key...
A cursory count puts the number of rooms in the neighborhood of 120. Gary's key has 18 numbered lines. Additionally, there are two color codings (pink and green).
At least some keys correspond to multiple rooms. It looks like seven or more rooms are labelled "2".
If we imagine that each key corresponds to seven rooms, that accounts for every room on the map. (I doubt that's actually the case, and clearly some rooms are unkeyed.)
Even if only one key corresponds to one room, that would be about one in six rooms keyed. If each key corresponds to 2-3 rooms with non-keyed rooms empty, that's close to U&WA ratio of empty to non-empty rooms.
Cyclopeatron's observations about running Arneson's Blackmoor Dungeons may be instructive:
"[...] the keys have virtually no information beyond what treasure and/or monsters might be in a given room [...] the generally empty nature of the dungeon causes an interesting tension to build. Empty... Empty... Empty... Empty... Screaming, confusion, blood, fire."
I don't see anything to make me think the page in the picture is not the complete key for the facing map, particularly if checks for wandering monsters happen every turn.