Hello all. I am in the US and am dealing with this document for a family member. I am a collector but this is outside of my experience.
This appears to be an original 8 January 1778 copy of the service letter (or commission or warrant or whatever it's called) from George III to Kenneth Mackenzie, Earl of Seaforth, to recruit a Highland Regiment of Foot.
My limited research indicates it represents a regiment that would eventually become the Seaforth Highlanders. It was presumably created in response to the recent loss at Saratoga and with the knowledge that the French might be close to joining the Americans (which they did in March 1778).
The regiment was designated the 78th or Ross-shire Regiment of Highlanders. In 1783 (or 1786) it was renumbered the 72nd (Highland) Regiment of Foot. In 1823 it was renamed the 72nd (Duke of Albany's Own Highlanders) after Frederick Augustus, Duke of York and Albany. In 1881 it was combined with the 78th (Highlanders) Regiment of Foot (the Ross-shire Buffs, which was formed in 1793 by Francis Humberston Mackenzie, a later Earl of Seaforth) to form The Seaforth Highlanders (Ross-shire Buffs, The Duke of Albany's).
While trying to educate myself on its history, I have found a few sources, and see another is posted in this thread
I hope that some find this of interest.