Route 74X was a seasonal express route between Baker Street Station and London Zoo during the 1980s. It was operated by a wide range of vehicles, sometimes on loan to London Transport, and the “E” plates would have only been found at either Baker Street or the Zoo, as buses did not stop intermediately. It is therefore an astonishingly rare “E” plate. In any case, Express “E” plates are quite uncommon.

 Thanks to the excellent website eplates.info for this information (link: http://eplates.info/074s.html#74X )