I have another data point on the frequency response. Look at the data sheet for the coles 4038: http://www.wesdooley.com/pdf/4038C.pdf
The frequency response is flat to 10kHz then drops steadily to (what looks like) beyond 40 kHz. There is no "null". To me, this suggests that in this case 1.) something other than the ribbon its self is limiting the high end frequency response and 2.) the frequency response limit of the ribbon its self in this case must be very high.