From Graham King. (2000). Good Punctuation. p.69.
In some cases, especially with names, we have choices according to taste. We can add the final 's (Tom Jones's songs,Prince Charles's opinions) or drop it ( Wales' ruggedness, Dickens' characters,Jesus' teachings). And there are many cases which, for one reason or another, observe rigid tradition. For example it is Queens' College, Cambridge, but Queen's College, Oxford.
Should it be "For example it is not Queen's College, Oxford, but Queens' College, Cambridge." ?
If so, 'but' in the original sentence should have a negative meaning.