There are.many clues this character is drawn from Jimmy Dale the Grey Seal - the character's name, the fact that he knew how to open safes because his father manufactured them, the fact that he left a small piece of paper behind as JD left a small adhesive grey triangle used to seal envelopes behind, his having an identity as a low level crook so he could ha NH out in the underworld. These are all exclusively JD characteristics. I have read a ill five JD books many times.
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The Shadow 3 (series 1)