Having concentrated a lot in recent months on various Territorial Force battalions, I'm going to look at regimental numbering in Special Reserve and Extra Reserve battalions. As I have written before, the Special Reserve and the Extra Reserve were the natural heirs to militia battalions which, for the most part they replaced. My 2009 article on the Creation of the Special Reserve in 1908 may be helpful here.
As far as the Scottish Rifles was concerned, its new 3rd (Special Reserve) and 4th (Extra Reserve) replaced the 3rd and 4th Militia Battalions and the new battalions just continued with the same regimental numbering systems that had been used by their militia predecessors.
Remember, the men joining the militia and special/extra reserve were not career soldiers. They were men who had completed a period of training alongside career soldiers at the regimental depot but they then went back to their homes, turning out annually for drills and camp. Men of the Special Reserve signed up for six years' service and knew, in the event of war, that they would be called upon to fill gaps in the regular battalions. A glance at most 1914 Star rolls will show plenty of evidence of Special Reserve men who served alongside career soldiers. Sometimes their numbers are prefixed by the number of their battalion - typically a 3/ or 4/ in most line infantry regiments - but just as often there will be no prefix.My summary below shows enlistment into the 3rd (Special Reserve) and 4th (Extra Reserve) Battalions of the Cameronians (Scottish Rifles). All of these men would have lived locally, within the recruiting area for the Cameronians.