An Index of Personnel for 9th Royal Scots is here https://neillgilhooley.com/9th-royal-scots/index/
The serial number issued to members of the 9th Royal Scots came from four or five distinct numbering systems. When the battalion was formed in 1900 it issued numbers as part of the Volunteer Force. Few of these numbers have been found:
The following graph is adapted from one published in ‘A History of the 9th (Highlanders) Royal Scots’, please see Appendix D for more information. It shows the new numbering from April 1908, when the battalion was reconstituted in the Territorial Force, to the end of 1916.
When members of the Volunteer Force joined the Territorial Force in April 1908, there were naturally a lot of numbers issued in the space of a few days, but the battalion only achieved half-strength. This rise was repeated in the intense recruitment of March 1909, as described in the book. Initially the serial numbers closely follow the battalion strength. Thereafter men joined and left at a fairly steady rate, about 150 a year, until war was declared. The earliest wartime serial number recorded thus far is No.1818 (4th August 1914). Recruitment in Manchester gives a block starting in November 1914 around number 2658. The rate of recruitment slowed in 1915, until late in the year we can see a large number of men attesting under the Derby Scheme. These men were placed on the Army Reserve but not mobilised (and given their battalion numbers) until called up in the following months. Conscription began in March 1916 with the Military Service Act.
An example of a draft transferred to another regiment is of those sent to 2nd King’s Own Scottish Borderers, most of these men were in the range 45xx-48xx and became a block 40877-40895 in 2/KOSB (Medal Roll Piece 1116).
From the beginning of 1917 the original battalion TF numbers (1-4 digits) were reissued as regimental 6-digit numbers, for the 9th battalion this was a block 350001-375000. These two graphs give i) dates for men with six-digit numbers (less sequential than the original numbering) and ii) a cross-reference between a man’s old and new numbers.
Five-digit numbers are not original 9th Royal Scots; but over a tenth of the battalion’s dead were 5-digit men drafted in without renumbering. Many of these would have been from 3rd Royal Scots.
Post-war the Territorial Army employed 7-digit numbering, the Royal Scots given 3044001 to 3122000 (Army Order 338 of 1920), and the battalion was amalgamated to become 7th/9th Royal Scots.