First RSK (Rootstock) merged mining

RSKBLOCK:
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RSKBLOCK: — Rootstock (RSK) merged-mining tag

RSK (Rootstock) is a Bitcoin sidechain secured by merged mining. Its mainnet launched 2018-01-02, and this coinbase at block 506,612 — about four weeks later, when the first pool began merge-mining it — carries the earliest 'RSKBLOCK:' tag on Bitcoin. RSK is the only merge-mined sidechain with a distinct ASCII marker, so unlike the opaque fabe6d6d AuxPoW commitment it can be identified on sight. The tag may sit in the coinbase scriptSig or, as here, in a coinbase OP_RETURN output.