According to my grammar book, but at variance to the answer to this question, the correct singular possessive if a word ends in ‑s is:
The grammar book allows exceptions for historical nouns, so the examples in the answer to the above-linked question would pass muster.
However, I’m sure that I learnt at school (which, admittedly, was a while ago) that for a singular (proper) noun ending in ‑s, the apostrophe went after the s and there was no additional s.
I don’t wish to start a flame war on which is correct, though my question doesn't really make sense if my grammar book is wrong! What I’m curious about is when the change occurred. :
So my question is when did James’s become the correct form and James’ the incorrect one?