When their replies came back, I noticed the handwriting was odd on the scoresheets. A quick check revealed Alan's move came back with Andrew's address card in the envelope, and vice versa. Clearly they had somehow sent moves for each other.
A few years later I happened to meet Andrew in an over-the-board game. He told me they knew each other (and played for the same club). A quick phone call and they had told each other what moves to send.
Anything else would have meant paying extra postage. Nobody ever wanted to do that.
Any other stories from the old days?