The account numbers sought in 21 and 22 are those of your bank's customers. There would be no customer accounts involved in your example.
However, if you were attempting to locate the supporting documentation at some future date the absence of an account number would suggest you had cashed a check that was not drawn on your institution. Obviously, that would send you on a wild goose chase in looking for the documents.
If you put in the account number or GL number for your cashiers check account, it would aid you in locating the documentation for the transaction at some future date.
I've spoken with FinCEN about this issue on both the FinCEN CTR and the legacy CTR, they said it was permissible to include the account number or GL account for the cashiers checks.