Seasonal housing-market factors pushed GSE single-family mortgage business higher in the third quarter, according to a new Inside The GSEs analysis of mortgage-backed securities data.Fannie Mae and Freddie Mac issued $213.81 billion of single-family MBS in the third quarter, a 10.4 percent increase from the previous period. The gain came from a 23.4 percent surge in purchase-mortgage business, which offset a 15.5 percent downturn in GSE refi activity. [Includes two data charts.]