First RSK (Rootstock) merged mining

0000  52 53 4b 42 4c 4f 43 4b 3a 1c c1 9c 8a 0e 06 b5
0010  0a b0 5f 2d 85 f4 76 54 08 45 74 43 e7 b1 51 41
0020  c3 85 bf e3 4a ec cc 60 5e aa 21 a9 ed 80 53 82
0030  45 5b 67 db ca 4b ef a8 02 0d ad d3 ba 9b 4f f0
0040  d2 8d ac 61 49 0c e7 7c 67 8f e7 8a 80

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.