The unconfirmed rumour I've heard is that there were six C-17s on the line with RAF serials allocated. These were more recent and additional to the four which were built for the lease. It is further suggested that only four of these six are still going down the line with UK identities, but that they are being built for direct sale, and not for lease.
Don't shoot me, it's not my rumour and I make no claims as to its accuracy, though I do believe that the exact serial numbers have been bandied about by the spotting fraternity for some time.