For everybody who is still looking for reference 5 in Peter Baxandall's 1963 Wireless World article, here is "Noise in Condenser Microphones" by Becking and Rademakers.
Regards
Attachments
Noise in Condenser Microphones - A.G.Th. Becking, A. Rademakers, ICA 01 Proceedings Delft 1953.pdf