Hi There,
The 5th battalion was a Territorial Force battalion. Generally speaking when war broke out those territorials who volunteered to serve in France became the 1st/5th Battalion and those who remained in England became the 2nd/5th of the particular TF battalion. The King's had the following TF battalions: 5th, 6th, 7th,8th (Irish), 9th and 10th (Scottish Bns. Also, I understand that when some TF battalions raised more men they formed 2nd and 3rd battalions of their battalion which became for example the 2nd/6th and 3rd/6th Bns of a Regiment. Bit crazy but thats bascically the explanation. The Kitchener battalions took their numbering after the TF battalions for example the 11th Bn The King's Regiment and were known as Service Battalions.
Regards
Chris