Dan-Air bought a 'job lot' of Darts from the MoD from recently retired Argosies. These were fitted to the Dan-Air sirs 2s to become 2As. The engines from the srs 2s were then fitted to the srs 1s, and the aircraft given the unofficial designation srs 1A.
(If my ageing memory serves me well!)
jn14:6, ex- DA 748s & Comets