Prepayments increased overall in September, particularly on agency fixed-rate MBS, with faster pay-downs occurring in lower coupons, according to analyst reviews of prepayment speeds. The experts expressed surprise at unexpectedly high prepayments for recent low coupon vintages and greater weakness for higher coupons. Deutsche Bank analysts reported that speeds for 4.0 percent Fannie Mae MBS issued in 2010 and 2009 more than doubled in September compared to the previous month. Speeds for similar MBS with 4.5 percent coupons increased also as much, they noted. For example, prepayment speeds for 2010 Fannie MBS with a 4.0 percent coupon...