Materia Hieroglyphica was published in Malta, not Manners and Customs.
The year of publication usually given for Manners and Customs is 1837, but it was actually published in January 1838: we can pin this down by advertisements.
The ‘-s’ (or ‘-is’) is a Greek terminal, just like Wilkinson said. It was perfecly normal to add it in Grecianising a name, as in ‘Thutmose’ to ‘Thutmosis’.
Here’s someone working up a conspiracy theory who can’t even read properly. Hopeless.