Coz thats how Rover denotes their engines. I think the original was the A series (minis, metros etc), i think some old MGs got the B series. The Maestro/Montego and some others had an O series (maybe that was a diesel?). Rover 200/400/600 had K, M, T series. (that was all guesstimated BTW) Dont know how the letter is actually chosen, go and use google for once.