George V sixpence difference

I have a 1926 George V sixpence, how can I tell whether it's KM815a

https://en.numista.com/catalogue/pieces21938.html

or KM828

https://en.numista.com/catalogue/pieces2344.html

What's the difference between the two?

  Modified Effigy
Similar to that of the Penny, and others. I posted pictures a while ago for a silver coin that I also had - will look tonight for the topic. It is easier to find by looking at the : colon between words - I think BRITT:OMN from memory. Not sure - but I had a quick look just now at the Numista images from those links you gave, and if yours is not too worn you can tell the difference by looking at the letter I in BRITT as on the first coin it points at a border tooth; in the second coin it points between them. Again, will look tonight ...  

Did some digging and one site mentioned that the initials were moved back onto the neck for the 1926-1927 coins.  I can't see initials on the neck of my coin so it must be the KM815a

https://en.numista.com/forum/topic28584.html

   That is the topic - which I see is 3 Pence, but I guess would be the same. Will make some 6 Pence pictures in a moment. But the BM initials not being seen could still be either coin, as both versions still wear down over time. The original version had B.M. (with dots) in the middle; the later became BM (no dots) on the right - like you said.

                             
 I made those just now with two 6 Pence I have - I chose 1925 and 1927, so guaranteeing those were different versions - rather than 1926 in case I had the wrong sort. I also see the later version has longer teeth (I compared to other before/after dates as well).  

 Here is a page that shows where BM is in both versions, and agrees with the longer teeth ...
http://www.treasurerealm.com/coinpapers/england/FAQ/oe_vs_me_12d.html

Thanks so much for that Zac. I'll have a good look tonight when I get home.

Excellent and comprehensive reply Zac. You deserve an award for all that you do.

On closer inspection mine is definitely KM828, which is annoying because I had a worn copy of a 1922 which I sold thinking I now had a better copy.  Now I'll have to hint out a copy of a KM815a