I just measured and indeed it is 2 - 2 1/2 inches from navel to bottom button on both of my jackets.

Jock, until I looked at your jacket I never took notice that some Argyll jackets have two buttons and some have one. Both my tweed, maded in Scotland decades ago and a navy Braemar I purchased new within the last ten years from Higgins, made in the USA, have two buttons. I cruised different Highland wear purveyors on the web and see that depending on the seller, either or both styles are displayed.