Because a returned letter must be associated with an account / account holder to be processed.
Though they knew this information when they sent it to the author, presumably it would be laborious to manually associate the same information with each returned letter (one would have to look it up anyway), so they probably print the data on the letter that may someday be returned, to allow quick lookup in the event it is returned.
It's equivalent to a conversation ID and interface crafted to avoid lookups, making this letter exchange idempotent, which I very much appreciate.
Why it was not requested to be returned is beyond me, but likely all such letters contain this.
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