In traditional Highland Dress as it was worn through most of the 20th century the wearing of flashes was standard, nearly universal.

I have several Highland Dress catalogues from the 1920s up through the 1950s and I don't think there's a single illustration of men without flashes, in either Day Dress or Evening Dress.





But evidently the situation was different back in the 1860s. In The Highlanders of Scotland are portraits of 56 kilted men, and 40 have no visible flashes.