Sources

Altas Mòr: 1^st edn OS map

Altas: Robertson [headform is Altasbeag]

Alltais: Watson in Dwelly and CW12, 2a [in list of names with the suffix -es]

G. Alltais: Watson 1904, xlix

V Altas: ‘altas’: ‘alt’ as in G alt ‘joint’ and as pronounced as in Eng ‘ash’, cu-ach tree (Sorry it is not pronounced “allt’.  CW11 [postcard to Watson from informant]

It seems Watson’s interpretation of Alltais from allt  + ais is wrong in this case, as in evidence with Robertson’s form, the OS form and the correction sent to him