Since the introduction of the single-security effort, pricing differences between Fannie Mae and Freddie Mac MBS have just about been eliminated, thanks in part to prepayment speeds converging. A paper recently published by the Urban Institute found that back in 2012 and 2013, Freddie’s 3.0, 3.5, and 4.0 percent coupons traded at more than a $0.30 discount to Fannie Mae’s. But, by 2014 and 2015 that number narrowed to about $0.15, and by early 2017 the pricing differences had largely come together. The authors called...