Lehman Brothers’ workhorse Structured Asset Securities Corp. ranked as the most active shelf registration in the non-agency MBS market during the first nine months of 2004, piling up $50.97 billion in issuance in a variety of different products. The SASCO shelf accounted for 8.3 percent of total non-agency MBS issuance through the first three quarters of 2004, according to a new Inside MBS & ABS analysis. Lehman used the program mostly for its heavy subprime