The uniform changed officially in 1937, but it didn't reach some second line units until after Dunkirk.
The inverted chevrons on the left lower sleeve are indeed the GCC.
Most Regts took from all over....I had a friend from Northampton, with no Scots ancestry at all who joined the Black Watch! Admittedly this was in the late 80s, but as the Regts moved around postings, they picked up recruits.
The 1st bn were at these locations...
1919 Scotland: Fort George
1920 Ireland: Belfast
1924 England: Aldershot 2 Bde
1928 Dover
1933 Egypt & Palestine
1937 China: Hong Kong
1938 Shanghai
1940.08 Hong Kong
1940.11 Malaya
1941.02 India
And the 2nd bn...
1922 Jan, Meerut
1924 Peshawar
1927 Lahore
1929 Meerut
1930 NW Frontier
1932 Palestine
1934 England: Dover (12 Bde)
1937 Scotland: Glasgow (15 Bde)
1939 Sept, France & Belgium (5 Inf Div, BEF )
1940 March, France & Belgium