orthography - When did it become correct to add an “s” to a singular possessive already ending in “‑s”?

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:

James’s car

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?

Answer

Since, 1810, forms like James’s (which I will call type A) have generally been more commonly used than forms like James’ (type B), according to my research using the Corpus of Historical American English (COHA).

I compared a number of names ending in -s looking for possessive forms with and without a final s. Here is a graph comparing incidences of type A and type B forms:

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The y-axis shows the difference in incidence between types A and B. If it is above 0, that means that type A was more common in that period; if below 0, that means type B was more common. The raw data used to generate the chart is in this Google Spreadsheet.

Here’s the same data, shown cumulatively:

For the 14 names tested, type A has been more common throughout the period beginning in 1810, except for the decades starting in 1850, 1940, and 1950.

Overall, it is quite clear that type A forms (e.g. James’s) predominate, and have done so for nearly two hundred years. Nevertheless, type B forms are also quite common, and during the 1930s to the 1960s, a number of names had more incidences using type B. But since 1970, most names have had a majority of usage in type A. As for the original question’s example of James, throughout almost the entire period, excluding 1820, incidences of James’s has strongly outnumbered incidences of James’.