Heavy purchase-mortgage business boosted Fannie Mae and Freddie Mac production of single-family mortgage-backed securities during the third quarter of 2017, according to a new Inside The GSEs analysis and ranking.The two GSEs issued a total of $223.61 billion of MBS during the three-month period ending in September, a solid 17.9 percent gain from the previous quarter. It was the strongest quarter of 2017, but production during the first nine months of the year remained 6.4 percent below the same period in 2016. Most of the gain came from the seasonal purchase-mortgage side of the business. Fannie and Freddie securitized $140.75 billion of loans used to acquire a new home, a huge 26.4 percent increase from the second quarter.