It's familiar from various live acts of the time, and has a distinctive housing, but I've never known what it is:
https://upload.wikimedia.org/wikiped...rt%2C_1976.jpg
Not sure because he has a tendency to wrap his hand around the business end, but Plant may have used the same mic in "The Song Remains the Same":
https://screenmusings.org/movie/dvd/...e-Same-349.jpg