Issuance of the net interest margin securities sagged in the early part of 2006, a period that saw the subprime MBS market cool a bit from the gangbuster pace it set in 2005. According to data compiled by the Inside Mortgage Finance MBS Database, $2.97 billion in new NIMs, or securities backed by such residual cashflows as prepayment penalties and late fees, were issued during the first quarter of this year. That marked a…