I've been told by a coupla different people that an SM57 uses the same diaphragm as the 58. They test the batches and the ones that don't spec out as well become 57s. Sounds far-fetched but like I said, I've heard it from more than one source. Is this just a myth? It would sure be embarassing to those who say the 58 is better for vocals and the 57 is better for amps/instruments.
Also I have a coupla Shure mikes that are shaped exactly like SM57s but they have PE66L written on them instead. What's the deal with these?