An unwritten possessive

In Montserrat in the West Indies (where I have just been) there is a village whose name is shown on the map as Frith. Yet everyone pronounces it frɪts. Why should this be? It certainly surprises outsiders.
Many place names in Montserrat are taken from the names of former sugar estates identified by the surname of the erstwhile owner: Blakes, Brades, Brodericks, Delvins, Drummonds, Dyers, Farrells, Gages, Geralds, Lees, Nixons, O’Garros, Trants, Tuitts, Webbs, Whites. The names of these villages obviously consist of the surname plus the possessive -’s ending. Occasionally they are written with an apostrophe, though more often not. The name of the village of Molyneux ˈmɒlɪnjuːz conforms to the same pattern.
Some names pronounced like possessives are nevertheless written without the -s ending. The village shown on maps as Farm is actually pronounced faːmz. Streatham is ˈstratəmz [sic]. Omitting the possessive ending in writing is particularly usual in the case of stems ending in a sibilant. There are (or rather there were, before the volcano disaster of 1997) villages called Harris and Weekes. But they are/were generally pronounced ˈharɪsɪz, ˈwiːksɪz.

The estate formerly belonging to Mr Frith should therefore be Frith(’)s. However in popular Caribbean English there is no θ: the fricative of BrE and AmE is replaced by a plosive t. So frɪθ becomes frɪt. Add the possessive ending, and we have the actual pronunciation frɪts.

Spoken Caribbean creoles are generally pretty cavalier about the possessive ending: you often hear things like Mary mother for standard Mary’s mother. But anyone who has been to school knows that you mustn’t omit the ending in writing. So it is all the more surprising that in these names people pronounce the ending but don’t write it.