The change to 2,5,12 etc was as late as the Royal Warrant of 1906. One reason for bringing the 6 forward was retention ....... as a result of the Boer War there were a lot of 3 years colour, 9 years reserve men who needed encouraging to stay on. The periods for badges changed almost as often as the army changed its socks!
1906 to 1922: 2,5,12,18*,23*,28*
In 1922 soldiers could earn more badges on top, at the rate of one for each 5 years. In 1961 the periods were reduced again.
The awards at the * items could be two years early if strict conditions were met.
By 1906 only soldiers with reserved rights were still getting the penny per badge, which began to be withdrawn in 1903 when Service Pay was introduced.
GCBs are one of my subjects! I have every relevant Royal Warrant and amendment from introduction in 1836 up to date.