returns you to the Base Text
For purely symbolic reasons an editorial change is made right at the beginning of the text, namely to remove the word حائط from the list of examples of nouns. There are far more substantial variations in the manuscripts and this is only an opportunity to display the possibilities of the Demo.
According to Geneviève Humbert, Les voies de la transmission du Kitāb de Sībawayhi, Leiden, New York, Köln 1995, p. 57f, حائط is found as the third example of a noun only in manuscripts belonging to the recension of al-Zamaḫšarī, hence those modern editions which include it are preserving a minority tradition. The Girgas edition does not have this word, obviously, because it is not found in MS B or MS C (access them from Girgas).
The reasons for the extra example are not clear: was it perhaps intended as a kind of marker to prevent unauthorized transmission (see M. G. Carter, in D. C. Greetham, ed., Scholarly Editing, a Guide to Research, New York 1995, p. 555f)? It is a remarkable fact that the manuscripts in the recension of al-Zamaḫšarī all omit the same seven words in the final sentence of the standard text, hence, (Humbert 104), the Zamaḫšarī version can be recognized immediately by its first and last sentences.