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 previous order, with the lowest numbered men recieving the first of the new batch.
240001 went to 5 Pte. Allan Christie (later awarded the D.C.M.). Christie attested on the 3rd of April 1908, shortly after the creation of the Territorial Force from the old local militias.
241258 went to 3842 Pte. James Forbes. Forbes attested on the 16th of November 1915.
On the 15th of March 1916, almost a year before the new numbering regulations were to be in place, the 1st/5th Btn amalgamated with the 1st/4th Btn to form the 4th/5th.
Looking at CWGC post amalgamation casualties, it is interesting to see there's mixtue of old and new soldiers numbers jumbled together in a 7 month period, starting from 03/09/1916 when the first renumbered man is recorded, until 01/04/1917, a month after the new numbering was supposed to be in use the final casualty was recorded using an old number.
Of 155 other rank casualties in this period, 101 are recorded under their old number with 54 under their 6 digit one.
Considering the even application of the new numbers to the old, it's odd that there's not a clean cut off where the new numbers take over in the casualties on CWGC from the old.
There's no pattern to the two number groups in this time frame. Neither is based on the previous Btn. a man belonged to, 4th or 5th, or if he's remembered on any particular memorial, or has a grave.
So why there's a large overlap in usage of the two numbering systems remains, to me at least, a mystery.