Class Z Reserve was authorised by an Army Order of 3 December 1918. There were fears that Germany would not accept the terms of any peace treaty, and therefore the British Government decided it would be wise to be able to quickly recall trained men in the eventuality of the resumption of hostilities. Soldiers who were being demobilised, particularly those who had agreed to serve "for the duration", were at first posted to Class Z. They returned to civilian life but with an obligation to return if called upon. The Z Reserve was abolished on 31 March 1920.
Interesting reading....

I think you will need the first wife's death certificate...
last posting looks like 47(S) RGA
I'd also have a dig on the 1911 census for margeret hutchinson....
interesting his first wife was known a s Hatton OR spence...
I suppose you have got the bit about BEF 25 feb 1916?
Also looks like he would have been one of the last tranche of Derby Volunteers prior to conscription in 1916