Writing in 1899 in The British Army (subsequently re-published by Greenhill Books as Scarlet into Khaki), Lieutenant Colonel James Moncrieff Grierson writes,
"The militia reserve is not, what its name seems to imply, a reserve for the militia, but for the regular army. It consists of militia-men whose number must not exceed a quarter of the establishment of a battalion of infantry, or a third of a battalion of garrison artillery, and who bind themselves, in return for a bounty of £1 a year, to remain with the militia either six years or the whole time of their service. In case of war they enter the regular army on the same terms as the army reserve men and can be employed in every quarter of the world. On entering the militia reserve the men must be between 19 and 34 years of age, and must have passed through two drill periods of the militia; they are liable to be called out to a yearly drill practice of 56 days. Service in the militia reserve cannot be extended beyond the age of 34. If called out on continuous service they are to be regarded as regular soldiers, and are discharged earlier or later on the same terms as the men of the army reserve. In times of peace the men of the militia reserve stand on the same footing as the other militia men and join in the yearly practice of their militia district."
The italics in the paragraph above are mine and explain why some oddities in regimental numbering can appear in Second Anglo-South African War (Boer War) medal rolls (when the militia had been mobilised). Take this example from the 2nd Hampshire Regiment Queen's South Africa Medal roll:
The regimental numbers for 2212 Lynch and 9548 Leary are not Hampshire Regiment numbers at all, they're the men's militia reserve numbers for the 5th Leinster and 4th East Surrey Regiment respectively.
Section 14 (2) of the Reserve Forces Act of 1882 deals with the Militia Reserve and explains how this apparent anomaly in regimental numbering arises:
"A man called out on permanent service shall during his service form part of the regular forces and be subject to the Army Act 1881, accordingly, and the competent military authority within the meaning of Part Two of that Act may, if it seems proper, appoint him to any corps as a soldier of the regular forces..."
Again, the italics are mine. So both men were called out on permanent service and both appointed to a different regiment from the regiments they'd served with up to this point - "any corps" to use the language of the Reserve Forces Act - retaining their original militia reserve numbers in the process.