It's a lot of years since that format of callsign was in use, and this particular vehicle certainly never had it. The St Mary's (Southampton) SEU is "54 Echo Uniform".
Added by Ian Moore on 24 October 2007.
You're probably right. I'm afraid my knowledge of callsigns goes back to the 1980s when this vehicle's predecessor, the 'ET', was known as 954.
Added by Ian Moore on 24 October 2007.