Glidrose (Fleming’s book publishers) has never divulged this information. It seems likely that Elleston Trevor (aka Adam Hall, the Quiller novels), James Leasor (Dr. Jason Love novels), and Christopher Nicole (aka Andrew York, the Jonas Wilde novels) were on that list.
Like Mr. Gardner, all three have written series spy novels. All three wrote regular fiction and the first two had received some critical acclaim for them. Nicole/York`s spy novels were set mostly in the Caribbean, ala Fleming`s original Bonds.