From Radiotron Designer's Handbook, 4th Ed. 1953, p. 728, item 17.5:
"There is no 'optimum' cross-over frequency because the choice is necessarily a compromise. Where distortion is the principal criterion, a low cros-over frequency from 250 to 350 c/s will be adopted for standard groove 78 r.p.m. Where needle scratch is troublesome with 78 r.p.m. a high cross-over frequency of say 500 c/s may be adopted."
On page 730, under Practical recording characteristics:
"There does not appear to be any generally accepted definition of published recording characteristics."
In the two figures on this page, European characteristics show several cross-over points, ranging from 300 Hz (Decca ffrr)
to 600 Hz for EMI 78 rpm. Note that there is no pre-emphasis at all for the EMI discs, which are 'flat' up to 10 Khz.
American 78s, however, show a boost of ca. 12 db at 10 KHz ...