It's likely that this case was for carrying documents, given its dimensions. It's marked Capt. Sprot, of Riddell, Liliesleaf, Roxburghshire.
John Mark Francis Sprot, known as Mark Sprot, was in the Second Dragoons, The Royal Scots Greys. He joined the Greys in 1901 and went straight out to South Africa for the 2nd Boer War, where he saw action. In February 1902 he was promoted to Lieutenant. From 1909 to 1912 he served as Adjutant of the Lothians and Border Horse, returning for a year in 1915. On the 26th October 1912 he became a Captain in The Royal Scots Greys. He served with the regiment in France in 1914, returning home shortly after due to asthma but went back out to serve on the Western Front in 1915. He retired in 1922 with the honorary rank of Major.
This case would have been a personal purchase by Sprot, as indicated by his home address and the lack of a broad arrow mark. The interior is lined in a dark green with no dividers to the interior. The lock is a secure lever and retains its key. Circa 1915.