Above is Henry Arthur Slatter (1866-1942), who led the 72nd Seaforth Highlanders, 1911-14 and 1919-1925. Here he is circa 1913, standing on the steps of the Vancouver Courthouse, which is today the Vancouver Art Gallery. This photo was posted by "Bold Highlander" on "X Marks the Scot," where he also posted photos of Captain Jack.
The third musical brother was Albert William Slatter, bandmaster of the 7th London Fusiliers. I'm still researching him!
All the brothers were children of John Slatter Sr. and Mary Shehen/Shehan, married in Whitechapel, London, England, in 1859. Their other children were Mary Slatter (hubby's grandma, married to James Edgar Wood) and Adelaide Mary Slatter (married to James S. Baker), of more in later posts. Mary must have passed the family musical tradition down to her son, Edgar James Wood, who played piano and other instruments professionally for many years.
UPDATE, 2022: For more about the Slatter family, see my ancestor landing page here.