"The regular battalions, that is, the battalions populated by career soldiers, operate a single regimental number sequence. A new recruit is given his number at the Regimental Depot, spends up to three months training at the Depot and is then posted to the 1st Battalion which is stationed in the UK. After 18 months to two years he is then posted to the 2nd Battalion which is serving overseas in India. His posting from the Depot to the 1st Battalion, and then from the 1st Battalion to the 2nd Battalion does not affect his regimental number which remains unchanged."
Here's an example of what I meant - and you'll find similar examples awash in WO 97, WO 363 and WO 364.
Michael Hooper joined the Royal Dublin Fusiliers on 8th Jun 1894 and was given the regimental number 5089. He was posted to the Depot and remained there until posted to the Home Battalion (in this case, the 1st Battalion) on the 23rd August 1894. He remained with the 1st Battalion, which was then stationed in Sheffield, until the 10th February 1896 when he was posted to the overseas battalion, the 2nd Battalion. This battalion would have been stationed in Quetta when Michael Hooper joined it and his service record notes that he remained with the 2nd Battalion until September 1902 when he was posted back to the regimental Depot. In the intervening years, Hooper would have seen service in Bombay, Natal and South Africa, the battalion moving back to Ireland in 1902 (and becoming the Home Battalion) whilst the 1st Battalion, already overseas in South Africa as a result of the Boer War, moved on to Crete and Malta and took up the role of the overseas battalion.
Michael Hooper was transferred to the Army Reserve in October 1902 and on completing this period of reserve service elected to join Section D Reserve for a further four years. He was finally discharged from Section D Reserve on the 7th June 1910.
At no point during his army career, did Michael Hooper's number change. Why would it? He joined as a regularsoldier and moved freely between the two battalions during his army career. Whilst on the reserve, had he been recalled to the Colours, he would have retained his service number. However, from the moment he was discharged in 1910, his number would also have been discarded. Even if he walked around the block and decided to re-enlist, that number would not have been re-issued to him.
The image from Michael Hooper's service record in WO 363 (above) is Crown Copyright and reproduced by courtesy of the National Archives. Interestingly, the same papers (but beautifully preserved) exist in WO 97, so here's another version of the same document:
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