Written to the chain on 20 Apr 2014, block 296,884.
First DOCPROOF-prefixed OP_RETURN timestamp in the scan. NOT the first Proof of Existence proof — the service began in 2013 with an address-encoded method [[proof-of-existence-genesis]] and only switched to this OP_RETURN/DOCPROOF format once OP_RETURN became relay-standard (March 2014). Self-discovered; not yet chain-verified.
OP_RETURN explicitly marks an output as unspendable and lets it carry a small data payload. Unlike fake-address methods, it does not create a spendable-looking UTXO. That makes it the least deceptive standard way to place small data on-chain.
More about OP_RETURNEvery byte shown here was read from these transactions. Open one in a block explorer to check it yourself.
