Thrift holdings of mortgage securities remained essentially flat during the third quarter, with a slight shift in concentration toward structured products. Institutions regulated by the Office of Thrift Supervision held $146.94 billion of mortgage securities on their books at the end of September, up marginally from the $146.80 billion in the previous quarter. In the somewhat broader sample of all federally-insured thrifts, aggregate MBS holdings dropped 0.3 percent during the quarter, to $218.69 billion. …