Yeah, it's old, but there is not a lot of first hand info out there besides the product spec sheets that have been made over the years. The extra bit I mentioned about the coil in the 57/58 being copper clad aluminum (specifically) is something I got out of calling someone at Shure...but I think I have seen that info online as well. Which is why it "looks" copper, but it is not solid copper like the R45 cartridge.
Anyway, the main point I was originally making was about similar susceptibility to hum field across all decades of production, which is removed to a large degree with the upgraded TAB transformer... and a sonic difference that occurs as the foam and cloth ribbon ages AND breaks in from use (and maybe the diaphragm itself?).
I'm not leading a crusade or anything, just relating some observations. As far as ancillary points that have popped up, I'm sure they may be true.
I'm quite certain that the original version of the 57 used a copper coil. Originally the SM5X mic series were simply "professional" versions of the regular Uniodyne/Unisphere chrome bodied mics with a TV friendly non-reflective finish and a slightly higher price tag, sold through "professional" outlets rather than regular music stores. Sure started making aluminum coils in the late '70s or early '80s, at the same time they released two alternate mic series, IIRC called the SM4X and SM7X series, but I could be in error on the numbers. One of these series consisted of transformerless versions of the 57 and 58 on slightly shorter bodies and, IIRC, were the first Shure mics (of this general type, anyway) to employ aluminum voice coils - at least that's the first mention I recall seeing in the Shure product literature.. One series was black, the other light brown, and both had an odd finish that was slightly fuzzy ("velvet like") to the touch. Neither series lasted more than a couple of years.
You can tell the difference between copper and aluminum wire when you try to solder to it - it's much harder to solder to aluminum with normal Sn/Pb flux core electronic solder.
This information comes from disassembling and working on many Shure microphones during a period between the late '60s through the late '80s, not from official Shure sources, which I have found to be less than reliable over the years.
EDIT: I just did a bit of googling and it appears that Shure is in fact currently selling versions of an SM48 (found no mention of a current 47) and the 7x series does indeed to have not outlasted the '80s. I found some online literature, however the mics do not appear to be exactly the same as the original issue that I remember; the newer versions no longer have the velvet touch rubberized finish and no mention is made in the spec sheets of voice coil material.. Both series appear to be transformerless. Both appear to be aimed at the karaoke market and similar applications, whereas when first introduced they were targeted at small clubs and semi-pro rock bands and sold at music stores alonglide the SM5X series mics. The current SM48 is advertised at outlets like Sweetwater as being a lower cost, budget alternative to the 58. My guess is that's where the off-spec capsules are going (response is given as 55hz-14kHz.). MSRP is $62, street around $50-$60. It appearts to be the only surviving model between both the 4x and 7x series.
Why is it that I'm feeling kinda like a mechanic who used to specialize in Yugos?