Union Jack wrote "
Suggest you have a wee shoogle with Google and, to help you, the date the rules changed, and Royal Marines and Army officer ranks up to full Colonel came into line was 1 Jul 99.
"
I remember the day well, I used to deal regularly with a RM Officer at the time who was a Lt on the Monday, promoted to Capt on the Tuesday and then became a Major on the Wednesday (1 Jul 99) when RM aligned its ranks with the Army rather than the Navy!!
I'm sure that, under RN traditions, that there can only ever be one Capt on board so visting RM and Army officers are always referred to as Major when embarked!