BA use their 'FICO' computer system to manage their fleets, and this works on the last three characters of the registration. e.g. PEC (for G-BPEC), XAR (for G-BXAR) etc. Whenever they add a new aircraft (or fleet) they need to bear this in mind, and try to come up with something whereby the last 3 characters are (a) unique and (b) clearly identify the fleet, where possible.
e.g. 757's were IKA, IKB....then the PEA, PEB series. 747 Classics were DXA, DXB, etc.
(Edited by the spelling police!)
[ 30 July 2001: Message edited by: In trim ]