Washington Mutual late last month completed what’s believed to be the first securitization of excess servicing spread from non-agency mortgage loans. The deal, called Washington Mutual Mortgage Securities Corp. 2003-XSF1, encompassed 15 certificates with a combined notional balance of $175.4 million. According to Fitch Ratings, which gave the transaction a triple-A rating, the interest-only certificates are backed by servicing fees on some $6.76 billion of fixed-rate WaMu MBSs issued since October 22.