In December 1918 the Class Z system was introduced by the army. This system, which released huge numbers of men very quickly, created a reserve of soldiers who would be recalled quickly to the army if the armistice was to break down.
Many of the men who were discharged to the Class Z reserve had claims to pensions for disability - presumably an assessment was made that this disability would not prevent the man being useful to the army again in future.
The Class Z system lead to a new pension numbering format being used for these cases.
The reference under the Class Z system was in the format of Z / Corps or Regiment / Sequential Number of claim from Corps or Regiment / Surname Split.