D6?

     Of course D6 is an assumed identity of No.6, and XO4 an imagined contact in Hammer Into Anvil, both cause a curious continuity between that episode and Do Not Forsake Me Oh My Darling. You will recall Villiers code name being PR12, and the Prisoner's former code name ZM73. So I suppose No.6 could have signed that note to XO4, ZM73, No.2 would have been none the wiser, and No.6 would still have maintained his anonimity at that point.
BCNU