In this thread "Who has/had the oldest mined Bitcoin?"
https://bitcointalk.org/index.php?topic=5421158.0 someone posted that signature:
-----BEGIN BITCOIN SIGNED MESSAGE-----
1E9YwDtYf9R29ekNAfbV7MvB4LNv7v3fGa
-----BEGIN SIGNATURE-----
1NChfewU45oy7Dgn51HwkBFSixaTnyakfj
HCsBcgB+Wcm8kOGMH8IpNeg0H4gjCrlqwDf/GlSXphZGBYxm0QkKEPhh9DTJRp2IDNUhVr0FhP9qCqo2W0recNM=
-----END BITCOIN SIGNED MESSAGE-----
The message is an unused Bitcoin address. But the signer's address was used in January 2009 (~2 weeks after Bitcoin started).
The message contains an unused address. However the signature verifies against pubkey used in 1NChfewU45oy7Dgn51HwkBFSixaTnyakfj which is a used address that received reward for block 1018.
As signatures don't have timestamps, it is possible that this signature was created back then. I searched the net the whole day, couldn't find an answer. I know someone posted here on bitcointalk years ago, that he mined block ~2000-3000 and that was the oldest mined block mentioned here.
The signature above was created with an older key (address was used in January 2009). Could it be Satoshi? Did he sign that message?