The good old BBC have made many of their research and development reports available through their website, spanning 1944 through 1996.
Below are links to some of the reports that relate to ribbon microphones. In many cases there is a lot of interesting technical data. The rest of the articles can be accessed here.
1945
The Crosley ribbon velocity microphone.
1953
Lustraphone VR53 ribbon microphone
The design of the PGD and PGS ribbon microphones.
The EMI microphone Type 2351E.
Tannoy microphone Type MD. 422.
1956
Design of the PGD and PGS Ribbon microphone Part 2
1958
RCA BK5
1961
Reslo RB
1965
Beyer M160
(Thanks to Santiago Ramos for sending the links.)