178.— Payments made to a court clerk under an attachment of earnings order shall, when transmitted by the clerk to the maintenance creditor, be deemed to be payments made by the maintenance debtor so as to discharge—

(a) firstly, any sums payable under the antecedent order, and

(b) secondly, any costs in proceedings relating to the antecedent order payable by the maintenance debtor when the attachment of earnings order was made or last varied.