Looks like this letter is 1973, the book 1981. His ultimate point is that he wasn't exploiting the letter for attention. 8 years after the incident, I think it's safe to claim the potential for exploitation is gone.
EDIT: Oh, you mean you took him literally, that he didn't even have his own copy. Well:
"And no copies of this letter have been sent to anybody else. You now hold the only copy in your hands."
It sounds like this could imply that he could be holding onto a copy himself. Interesting point, why would you do that if you didn't mean to exploit it, at least eventually? Or maybe he just likes to hold on to things for records.
I know personally I keep copies of every letter I send, simply to refer back to if and when the recipient responds to me; it's useful to know what questions I asked when the answers come in.
That's a lot easier now than it was back then. Maybe he used carbon paper but there was no photocopying and obviously no word processing, at least nothing that an individual outside of a specialist organisation would have access to.
It's obviously the most likely explanation but still, it perhaps stretches what might be considered usual for the period.
EDIT: Oh, you mean you took him literally, that he didn't even have his own copy. Well:
"And no copies of this letter have been sent to anybody else. You now hold the only copy in your hands."
It sounds like this could imply that he could be holding onto a copy himself. Interesting point, why would you do that if you didn't mean to exploit it, at least eventually? Or maybe he just likes to hold on to things for records.