A February 1971 article in the Sounds paper listed Robert Plant as using Shure Unidyne IV 548SD Microphones. He was seen using the Unidynes through the 1975 Tour.
In 1973, Plant endorsed Shure Unisphere I (precessor of the SM58) and was seen singing through it on the 1973 US Tour.
Have a vintage model and honestly prefer it to the SM57 which is more common. This is the predecessor but it still sounds better to my ears. Check out the graphs on the Shure website to compare the two.