My grandfather was also in the 1/5 Argyll and Sutherland Highlanders during WW1, unfortunately it appears that his military details were part of the 70% of documentation destroyed in the blitz in WW2, so any info I have is passed down via the family.
The 1 in the 1/5 refers to the fact that this territorial battalion was a 1st line battalion ie always destined for front line duties, whereas 2/5 was a 2nd line battalion used for other duties and as reserves for the 1st line.
I would recommend two very interesting books relating to the time your husbands grandfather WW1 experiences, both show what an absolutely terrible time he and his army comrades endured , a fact many people do not know about, as the eastern theatre of operations was labelled a "cushy billet" by those fighting on the western front , on the basis that it was sunny and on not trench warfare(incorrect).
The first book is Forgotten Soldiers of the First World War by David R Woodward and the second is the history of his division from Gallipoli to the end of the war on the western front, this is simply called Fifty Second (Lowland) Division 1914-1918 written in 1922 by Lieutenant R.R. Thomson.
