As far as the Buffs was concerned, it was the 3rd (Militia) Battalion which it lost with the creation of the Special Reserve in 1908 and those concerned with such matters appear to have simply shrugged their shoulders and carried on with the same regimental number series that had been in use for the militia.
Men joining the militia and special/extra reserve were not career soldiers. They were men who had completed a period of training alongside career soldiers at the regimental depot but they then went back to their homes, turning out annually for drills and camp. Men of the Militia and the Special Reserve signed up for six years' service and knew, in the event of war, that they would be called upon to fill gaps in the regular battalions. A glance at most 1914 Star rolls will show plenty of evidence of Special Reserve men who served alongside career soldiers. Sometimes their numbers are prefixed by the number of their battalion - typically a 3/ or 4/ in most line infantry regiments - but just as often there will be no prefix.
The first 3rd (Special Reserve) Battalion number I have on my database - and there will be many earlier ones thatn this - is 7196 Ernest William John Brooks who had been issued with that number when he joined the 3rd (Militia) Battalion on the 3rd June 1904. He was still a serving member of the 3rd (Militia) Battalion when the Special Reserve was created, and he signed on for a further six years in July 1908, his old militia number being re-activated. Other men who joined at the same time and who were not already serving with the militia were issued with new regimental numbers which began where the old militia regimental series had ended. This pattern is seen in very many special and extra reserve battalion attestation papers for 1908.
Here then, are some sample regimental numbers for the 3rd Buffs (East Kent Regiment):