The history and reorganisation of the reserves during the past 125 years is an immensely complicated subject so I need to generalise a bit to stop this becoming too long.
The AER was a part of the overall reserves which existed between 1951 (created by the Reserve Forces Act 1950 although the words 'Emergency Reserve do not occur in the Act) and 1967 when the Reserve Forces Act 1966 came into force. They were also known as the 'Ever-Readies'.
The AER consisted of a small percentage of the overall Reserves (no more than 15,000 in strength) made up of specialists and technical trades who could be mobilized quickly without the need for a Royal Proclamation which was necessary to mobilise the main reserve forces. As a technical Corps, some former soldiers of the Royal Signals were often included in the AER. Members of the AER represented a central pool of manpower who would be shadow posted to specific regular units to bring that unit up to its wartime strength. Their commitment in the AER was for 3 years although they could extend this if they chose to. The training commitment was for an annual 2 week training camp, or a two week attachment to the regular unit to which they were likely to be assigned in times of emergency. The AER drew its manpower from ex regulars and ex National servicemen. They were paid a higher bounty than other reservists or the TA. The Suez crisis in 1956 identified a number of weaknesses in the composition of the AER, or more to the point, the fact that it didn't cover enough specialists, and consequently a Royal Proclamation had to be issued in order for the War Office to be able to mobilise the additional specialists it needed but who were not then represented within the AER.
I don't have a copy of the 1956 AER Regulations so I can't tell you exactly why he was released. The standard reasons were:
- Completion of the 3 year term.
- Voluntary ie at the request of the individual due to a change of personal circumstances, eg going to work overseas.
- Early release, ie the Army no longer wished to keep the soldier in the Reserve. In the case of the Royal Signals where soldiers required advanced security vetting, a loss of the vetting status might mean the individual was transferred to the normal reserve or his reserve commitment was terminated.
- Medical.
- Restructuring of the overall AER. In other words the individual's trade or technical ability no longer fitted in the priorities for the AER.
There were probably some other minor reasons, such as failure to attend the compulsory training without a legitimate excuse, failure to notify changes to contact details, etc.
Since your man only did two years in the AER he wouldn't have been released on completion of his commitment.
