Seeking Grammar Police Ruling - Typo's / that possessive 's

[Mike]

> > Geoff:
> > Standard UK practice is to add just an apostrophe to a proper
> > noun, so we would have Sirius' coat or James' wand... 
> > In Mike's example above using "Dents' car", 

Mike:
That's the way I was taught, Geoff. However, I am seeing an 
additional "s" after the apostrophe so often these days that I'm 
thinking the convention has changed.



> Potioncat:
> 
> And Mike, you knew what you were talking about with pronunciation. 
> Whether you would add 's to a proper noun that ends in s does 
> depend on pronunciation. 

Mike:
Wait,... what? I was right about something? That doesn't happen too 
often, pray tell how was I right? 

In speaking, I would pronounce it the "Dents car", I wouldn't say 
the "Dentses". Conversely I would pronounce it "Jameses car". Does 
that make it correct to uses -James's- while still sticking to the 
conventional -Dents'-?

Does someone have a better example? Or is there a new rule to use 
when applying this new convention?

Mike, extremely pleased to be right even if he doesn't know how it 
happened :D