Just to throw in my tuppence worth.
Barry Buddon camp near Carnoustie is still very much on the go. Forfarshire became Angus in 1928 so the letter is post that. Early 1930's would put him in the first world war.
I think the photo is inter war poss mid 30's. The tunic style is still ww1 not the shortened blousen jacket of ww2. I first thought it was RMP but the badge isn't correct. They are a bit of a mixed bag short and long puttees plus at least one with dark buttons- wouldn't be RAF Regiment by any chance?
David