In addition to soldiers who were long term sick, prisoners of war, AWOL or undergoing sentence at the Military Correction and Training Centre (MCTC), the Y list could be used for soldiers on terminal leave. This would appear to be the case with your granddad as he didn't join the W(T) reserve until 17 April 1946. The reason for this was that men on the Y list (as opposed to the X list) were totally excluded from the numbers of men available to fill establishments, even though they were part of the overall strength of the Army at that point in time.