First merged mining (Namecoin AuxPoW)

0000  04 5d ee 09 1a 03 27 26 01 52 2c fa be 6d 6d 84
0010  d7 bb 78 c6 68 b5 7a c9 97 ed 51 bc 19 5b b7 8f
0020  a4 f3 31 b7 34 62 ad ff d6 bc 33 ce 20 ac 64 01
0030  00 00 00 00 00 00 00

fa be 6d 6d — AuxPoW merged-mining marker (Namecoin)

The first merged-mining commitment on Bitcoin. Merged mining lets a miner secure a second chain with the same work, by committing that chain's block header inside the Bitcoin coinbase. Namecoin enabled it on 2011-09-19, and this block — 148,744, three weeks later — carries the first commitment to appear on the Bitcoin side. The magic bytes fa be 6d 6d (\xfa\xbe"mm") introduce a 32-byte auxiliary block-header merkle root. Because Namecoin was the only merge-mined chain at the time, this first marker is unambiguously Namecoin; every later one commits an opaque root that cannot be attributed to any particular chain from Bitcoin alone.