Werner's paper
Electrical Loading of Mics shows "walleys".
Journal Of The Audio Engineering Society
October 1955, Volume 3, Number 4
On Electrical Loading of Microphones
Richard E. Werner
Radio Corporation of America, Camden, New Jersey
"The series of lesser resonances at odd multiples of the fundamental are due to higher modes of vibration in the ribbon and are perfectly normal in a ribbon microphone. In high-quality ribbon microphones these higher-order modes are of a minor nature and do not detract from the excellent transient response and low distortion which are recognized as the outstanding attributes of the ribbon microphone. It is apparent, however, that these resonances complicate considerably the mathematical expression for the microphone's output impedance."
As far as I can figure, Olson was still hanging around RCA at the time this was published.