A phono preamp with equalization selector

You peaked my curiosity as to whether there is some new information pertaining to equalization curves used in the earliest decade or so of 33 rpm vinyl LPs.  (So this has nothing whatever to do with any record that is not in that category.)  You might be interested to read what Michael Fremer had to say on this subject, with documentation of his position:

Note he specifically mentions Deutsche Gramaphone LPs.

It is pretty much established that by the 1960’s everyone used RIAA.  But, nonetheless, there are quite a few records that sound better using some other curve.  I heard a demonstration where 1980s recordings sounded better with the old Columbia curve, for example.  I heard this in a demonstration of a Zanden phono stage.  I heard similar results with other stages with equalization choices, including an FM Acoustics phono stage with infinite equalization choices—it had two knobs, one to choose the crossing frequency and the other to pick the slope of the curve.  

Infinitely adjustable on a Parks Puffin

Of course that is possible, but do you think it really has to do with the "1980s"?  More likely that a single particular recording regardless of the time line, might by chance sound "better" using an equalization curve other than RIAA. (And Columbia is not far from RIAA, anyway.)  And besides, what does "better" mean? (It's a subjective judgement.) The point is that nearly all companies (I say "nearly all", because I don't doubt that someone can find an exception or two) used RIAA certainly by late 50s/early 60s.  I think Fremer is claiming RIAA took over even earlier in the history of the LP. In the early or mid-50s, my parents had a Harmon Kardon mono, tube receiver that offered RIAA, Columbia, and one or two other choices for equalization, but note the date.

My Audio Research Reference 6SE has Columbia and Decca. Used in the 40’s and 50’s as well as RIAA.