What is it, and how do I wear it?

[OC Richard]

It looks like a specifically Highland Evening jacket, to be worn with an Evening Dress belt such as is worn with the Montrose.

I knew it looked familiar- here it is! Being worn by the Alexander Brothers

https://www.youtube.com/watch?v=-Ng4XklfxsI

It strikes me as a blend between a Prince Charlie and a Montrose.

Here is Pipe Major Evan MacRae wearing a not dissimilar jacket, though his buttons higher. I took this photo c1981



The man on the left is Lt Col John McEwing, the founder of the North Idaho College School Of Bagpiping.

[D.A. Guertin]

Again, very interesting... Thanks very much for that! I'll have to see if there are belt loops or hooks once I get it in; couldn't see any in the photos I had available.