Another reason for the necessity of a variable speed drive, is that
many records were deliberately speeded up or slowed during recording,
and editing to make the performer sound faster, or hit higher notes,
or fit the time.

Many 33's also vary, and between tracks.  I learned by playing along,
and the constant retuning of my balalaika drove me mad, until I got a
variable speed turntable.  Records varied as much as a tone, but were
not in any increments, such as plus or minus a half tone.  Speed
control was as much art as science then, just like the player pianos.

I was told that European instruments were tuned to 400 cps, and not
to 440 cps.  Another difference reason.  Most electronic tuners have
switches to vary the standard to accommodate this.

Will Herzog
Rochester NY