It was the Army Council Instruction (ACI) 2414 of 1916, published on 23 December 1916, that among other things, ordered the renumbering of the men of the Territorial Force.
Previously numbered 1 - 9999, the Territorials were to be allocated a new (and in most cases) 6 digit number.
The changes were to be implemented by the 1st of March 1917.
In the case of the 5th Black Watch the number block given over to them began at 240001.
With few exceptions the renumbering followed the