A phono preamp with equalization selector

GHD, You say "used in the 40s and 50s".  For most of the 1940s, the 33 rpm LP as we know it today did not yet exist.  I think the very first such records were produced in 1948-49.  It was the introduction of this then new technology that necessitated the new equalization curves. 78s and whatever else was in common use through most of the 1940s were certainly not equalized according to any of these algorithms.  From there, I refer you to the article by MF that I referenced above. Whatever one might think of MF in terms of his opinions as an equipment reviewer, I do trust him to do a thorough job of research when he addresses these general subjects that apply to the hobby.  I take Ralph Karsten's (Atmasphere) word as well, that the companies that made the equipment upon which LPs were cut (Westrex, etc) who determined what equalization curve to use, and that all of these companies had adopted RIAA by the mid-50s. (Yes, I suppose there are rare exceptions, but I don't know of any, and maybe they did it differently at first, in the UK.)

@cleeds

Yes, my bad.

👍

@lewm 

Yes, you are correct the LP was introduced in 1948.

Right. I have access to every eq curve ever concocted. I only use RIAA except when playing old 78s. If you do not play old 78's it is totally unnecessary to consider other EQ curves. It is for all intents and modern purposes a marketing feature no one needs or uses.  

I'll Take You Down Memory Lane.

My Uncle's 1958 Fisher President II, EQ had not yet been Standardized

Very Unique 'Stereo Radio' (Live Binaural Broadcasts)

Look up the Harmon Kardon Festival monaural receiver. That’s what my parents had as of the early 50s, even predates the unit shown above. As I mentioned, it provided a choice of EQ curves including RIAA, Columbia, Decca, and maybe one other. I’m guessing that this is because the industry was just settling on RIAA during the period 1948-55(?). I do recall fiddling with the EQ in order to hear the differences. So maybe if you own very early production mono LPs, there might be a marginal benefit; the different EQs did not seem to make much difference to tonal balance, to my pre-teen ears.