A. They were allocated sequentially by date of enlistment (battalion or brigade - see below). So knowing the date of enlistment & number of two soldiers it is possible to guess the probably date of enlistment of a soldier in between the two numbers you know. However three series in use during AZW - 1st Battalion, 2nd Battalion and 25 Brigade.
B. Officers did not enlist - there were so few with common names - confusion only arose during WW1.
During the period 1856-1873, individual numbers were allocated to recruits by the depot (or recruit) company of each battalion of the 24th (2nd Warwickshire) Foot. The depot would normally be co-located with the battalion when it was stationed in UK or Ireland. When the battalion was abroad, its depot company would remain in the UK usually based in a garrison town. The established strength of each battalion was about 800 men. Separate series of personal numbers for each battalion of 24th. Thus No. 1 was issued by the 1st Battalion to Recruit Perkins in December 1856. When 2nd Battalion was re-raised on 3 June 1858, recruiting started at Sheffield; and by December 1858 number 593 was allocated to a recruit named William Jones (later of Rorke’s Drift VC fame). Each man enlisted for a period of 10 years with an option to re-engage at the end of the period. It can be seen that during this period there were soldiers serving in the 24th with the same number albeit in different battalions.
The 24th Regiment was finally given a permanent depot in the barracks at Brecon in 1873. Recruits came to Brecon for processing and training and each was allocated a 25th Brigade number (Brecon was then known as the 25th Brigade Regimental District). 25th Brigade Number No. 2 was allocated to Drummer John Orlopp (later killed at Isandlwana) in July 1873. This system eliminated duplicate numbers within the 24th. However at the time of the 1879 Anglo-Zulu war, there were three number series in operation, viz. 1-24th, 2-24th and 25th Brigade in the Regiment and all series were used on the regiment’s roll for the 1877-79 South Africa War Medal.
When the title of 24th Regiment changed to the South Wales Borderers in 1881, a new series of personal numbers for soldiers was introduced. This remained in force until 1920 when a universal series was introduced for the whole British Army. In the early years of this series, serving soldiers re-engaging with an earlier battalion or 25 Brigade number would be given a new number in the SWB series. SWB number 1 was allocated to Private Thomas King who enlisted on 2 July 1881.
During the 1880s, each Militia and Volunteer Battalion of the South Wales Borderers, introduced their own battalion series of numbering.