The notion of a common pass-through MBS issued by Fannie Mae and Freddie Mac was formally put in play this week, as the Federal Housing Finance Agency solicited comment on a proposed “single security” for the two government-sponsored enterprises. The Mortgage Bankers Association has been pushing the concept for several years, and it made it onto the FHFA’s formal agenda for the GSEs in 2014. Proponents say it would improve liquidity in the market and level out Freddie’s pricing disadvantage. Under the FHFA proposal, the new common MBS would adopt...