Men enlisting in WWI were issued with a regimental number by the AIF. Exceptions were officers and nurses, who were not issued a regimental number.
Numbers, while sequential, were rarely unique. The number “1‟ was allocated to the first man in every infantry battalion and light horse regiment. Therefore, a minimum of 20 men could share the regimental number “1‟ – one for each of the 16 infantry battalions and four light horse regiments.
When a soldier was transferred to another unit which already had that regimental number his number was appended with a letter. The numbers of re-enlisting soldiers often included the letter R.
With the formation of the General Service Reinforcements (GSR) in 1917, the numbering system changed; general reinforcement soldiers were allocated a unique number between 50,000 and 80,000.