I bought it to use with an SM57 and a cheap Royer R-121 clone—the t.bone RB 100. Despite their different diameters, the SM-21 fits both thanks to its hard rubber construction. That said, the holder lacks a bit of stability, especially under the weight of the mics. It still works, but positioning takes some care.
By the way, the SM57 and RB100 combo works great—no phase alignment issues, and the frequency response is exactly what you'd expect from pairing a dynamic with a ribbon mic.