The Federal Home Loan Banks are on pace to grow their collective holdings of mortgage-backed securities by double digits in 2004. That pace would shatter the rate of MBS growth recorded in 2003, when the Banks increased their holdings by just 2 percent. It appears, however, that there may not be much room for further growth through the end of the year. The $112 billion in MBS the FHLBanks held through the third quarter represents…