MMario,

Officers of the British Army, certainly at the begining of the 20th century, wore a "frock" (an unlined and unstructured jacket) in "tartan", these frocks were either scarlet or blue (depending on the arm of service), but were solid coloured and not patterned. I maintain that "tartan" is the cloth (Dress regulations refer to the pattern worn by The "Black Watch" as "Government sett" or "Government pattern" not "Government tartan").

Walrus