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.
