A pressure gradient cardioid moving coil microphone, which uses a 'phase shift' network to achieve the unidirectional response. It was ideally suited to sound re-enforcement applications. Like most pressure gradient microphones it is particularly prone to popping or blasting if worked too closely.
The second photograph shows a pair of 4105s mounted on a BBC STM23/B - part of a 'Royal' microphone stand kit. Each BBC region would have been supplied with a set of Royal mic stands and microphones, some table mounting and some floor mounting for use on Grade 1 broadcasts. Grade 1s were programmes of great national importance - for example, a broadcast by the Queen. For such events the whole programme chain was set up in duplicate to allow the speediest possible recovery from technical problems.