I know very little about this man. Chailey Parish Magazine mentions him once in its April 1917 issue, stating: Campkin, Pte H, TR Batt.
The following information about the formation of Training Reserve Battalions is taken from Chris Baker’s website, The Long, Long Trail:
On 1 September 1916, a considerable reorganisation of the reserve infantry battalions took place. Before this date, most of the infantry regiments contained one or more reserve battalions. Recruits would be posted to these battalions for basic training, before they were posted to an active service unit. With the introduction of conscription, the regimental system simply could not cope with numbers, hence this development. Thus, in combination with conscription, the local nature of recruitment for infantry regiments was abandoned.
After 1 September 1916, these regimental distinctions disappeared, and the reserve battalions were re-designated as battalions of the Training Reserve. They were organised into new Brigades of the Training Reserve. No Guards, Irish or Territorial Force Battalions converted to TR, and this change did not affect the Special Reserve or Extra Reserve battalions of the regular army (normally the 3rd and 4th Battalions of a regiment).
The official complement of the Training Reserve was a little over 208,500 soldiers. Men who attended the TR battalions were not allocated to any particular regiment when the time came for them to be posted. Thus, in combination with conscription, the local nature of recruitment for infantry regiments was abandoned. Later, from May 1917, this arrangement was itself altered when the units of the TR became Graduated and Young Soldier battalions.