In 1980, while languishing in DKMH Catterick, an old gentleman in the ward told me his number only had three digits. From memory he served at Catterick as part of the RFC/RAF during WW1
In WW1 the army did not have a system of unique service numbers until some time in 1916. Prior to that they had regimental numbers which in fact started at 1 and were duplicated across the regimental system. Thus there could be a number 456 in the Suffolk Regiment and another in the Norfolk Regiment, and so on. The same applied to corps, although as these tended to be larger than line regiments their numbers often contained up to five digits. The unique service number came in to assist in admin when the army grew in size to several millions. The old gentleman at Catterick would have lost his original three digit number when the new system came in.
In the medal collecting world medals to individuals with low numbers are highly prized. I once had 1914-15 Star named to a number 10, an NCO in the Royal Warwicks, and a chum who specialises in this field has several "number 1s" in his collection. Although army and RAF officers have always had service numbers, these never appear on medals named to officers (check your own!)