The primary indication is the similarity of the drawings themselves: same orientation, same arrangement of characters within the cartouche.
Secondary are the captions (“in Campbell’s Chamber” and “Cartouche in Campbell’s Chamber”) and the reference marks (slanted double dagger).
Creighton ignores the shorter caption (“in Campbell’s Chamber”) entirely, doubtless overlooking it in his haste to assert that this is a drawing of something Vyse found outside the pyramid.
You will already have noted the absence of “To be copied into Campbell’s Chamber”, “Forge this one” etc.
We may note also in passing that Creighton has quietly dropped the suggestion (vehemently asserted not so long ago) that the second character following the cartouche is “anachronistic”. This is typical of his arbitrary treatment of evidence.
Post Edited (28-May-14 13:12)