Just to follow on from the above. The backmarks you have listed highlight an important point relating to backmarks in general.
The first one is without "Ltd". Gaunts became a limited company in 1899 (at least I think it was 1899, I'm not at home and can't check - Roger will shout at me if it's the wrong date!
) It's easy to assume that because there is no "Ltd" then the button must pre-date 1899. Not so, they continued to use that backmark for many years, alongside those which included "Ltd".
The point I'm trying to make is that while backmarks can be very useful in dating buttons, it's best not to place too much emphasis on the exact wording.
David
Having said the above, it's certainly true that Gaunts didn't use "Gaunt / London" on large size buttons until long after WW1. There is much debate as to when it was first used, some would say as early as mid 1940's, others no earlier than c.1950. It certainly wasn't used on any genuine large size buttons of the disbanded (1922) Irish Regiments.