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I have no idea why not, but SG just do the 1901 issue, the London overprints and then straight on to the 1910 overprints. Strange, and quite an omission, one would think, but there it is! Or isn't!
This sound a lot like simplified "Stamps of the World" listing. SOTW listings omit everything that doesn't change the stamps design significantly... So you definitely need to digg out a copy of SG Portugal & Spain catalog somewhere and recheck.