There are four 'cages' spot welded to the underside of the upper
baffle, the nuts are loose and held in place by folding in the
four sides of the cage. So yes, they are a captive 1/4-20
square nut. The original screws would have been 1/4-20 x 1/2"
binding head clutch drive screws.
http://www.pbase.com/dennygraham/image/159522304dg