"Users of wide-range playback equipment should compensate for the RIAA curve"

The warning was probably on the original pressing as well, a notice to distinguish the disc from competing compensation curves, such as from Decca or Columbia.

When competing solutions exist, prior to 'standardization', i.e. RIAA was the as yet undetermined but eventual standard, the specifics of which solution was used, they are telling you it was recorded using the RIAA eq, and you should therefore use RIAA Playback curve.

LP means Long Play. Bass had to be cut during recording to minimize the width needed for bass notes, to get more grooves closer together, to get more music on each side, i.e. Long Play. 

I can choose from a bunch of curves other than RIAA. It's interesting to hear the differences.

I have a lot of mono reissues of albums from the 50s. Many have facsimile jackets with the same instruction to use the RIAA playback curve. It's true that just about anything cut since that period use RIAA equalization by default. The guidance to use RIAA for playback is just a relic of the era, when other equalization curves were also used. My phono-stage has RIAA playback by default, but it also has switches for EMI, Columbia and Decca (FFRR) curves, for those who have original records from the period mastered/cut with them.

It how it was mastered in the day.Great when you can adjust the sound some what to your liking.If possible.