Ok I think my original non-authorative assertion holds!!
I also looked through stellar-core and have confirmed in a round-about way via horizon. The interesting stuff is in ./src/xdr which is a set of C like files outlining the various interfaces used for the transactions, ledger, consensus etc. Cool stuff.
Anyway horizon testnet....
GET the operations associated with an account, ie.
GET https://horizon-testnet.stellar.org/accounts/<Account-Address>/operations
Find a "manage_data" key in the returned JSON and GET the associated "transaction" URL, ie.
GET https://horizon-testnet.stellar.org/transactions/<Transaction-ID>
Find the "ledger" number and GET the transactions associated ledger entry, ie.
GET https://horizon-testnet.stellar.org/ledgers/<Ledger-Number>/transactions
Find the account "source_address", transaction "id", and "ledger" confirming that everything matches.
Find and decode the "envelope_xdr" confirming the values set in "manage_data"