As far as modern Sc.G is concerned, I agree with Pòl.
The fact that a number of ordinary nouns ending in -s have occasionally also
acquired a -t in some areas suggests that there is more to it. Example:
And my feeling too is that it was phonological. I don't wish to start
straying off topic - but suffice it to say that I would need to argue for
speakers having invented hypercorrect forms.