Nowhere on XKCD’s map of The Contiguous 41 States does it say that you need to find the missing seven states, but that’s immediately where my mind went. And it was a little more challenging than I anticipated — all of New England is present & accounted for somehow?
The United States did have exactly 41 states for a few days in 1889, from the admission of Montana, the 41st state, on November 8, to the admission of Washington (the state, not DC), the 42nd state, on November 11.