They represented an attempt to regionalise some of the names of the standard pacifics, resulting in E.S.
Five of the unbuilt locomotives were expected to operate on the Southern Region and so a change in naming style was planned for the next five in series.
It was intended to revert to Scottish Region parlance for the final ten.
The names Hengist and Horsa had been used previously by BR for cross-channel Sealink ships.
A start has been made on constructing a new locomotive of the missing batch of 15, number 72010 Hengist, being assembled at CTL Seal in Sheffield.