The comment with this photo suggests that the hat was issued, for a short period, as a replacement for the glengarry

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"Although sadly, I cannot identify this soldier, he is a member of 1/5th Seaforth Highlanders. This photo was almost certainly taken outside his billet house in Bedford in April 1915. In the first winter of the war, the glengarry was found to be impractical, and so this rather strange form of headdress was one short lived attempt to overcome the problem. As the other photos show, the Tam O' Shanter bonnet soon took its place."

From: http://www.historylinksarchive.org.u...number1134.asp