Unless my Shure catalogue data is wrong, and I don't think it is, there are significant differences between the Shure 540 and 545--the 545 is NOT the same mic that Butterfield used, and is not like it. The differences include frequency response, output level, and polar pattern. The 540 is not a cardiod mic. It's nothing like a 545. Which isn't to say they don't sound good.
Another question. Andrew also makes note that the Astatic biscuit he has is the same mic Little Walter played. I'd like to see the interview where Walter specifically names Astatic as the maker of the biscuit mic he played (in fact, if I remember, he makes reference only to a mic "shaped like a biscuit"). Since at the time of his comment there were at least three companies making mics that fit this description, all with ample distribution in the upper Midwest, I think it's a bit presumptious to assume Walter was referring to an Astatic mic. Anyone have the proof Astatic was what he was talking about?