My friend, the retired newspaperman, says that the _contemporary_
American definitions (usages) are

•   disk: Astronomy -- (lunar or solar disk)
         Anatomy -- (in the spine)
         Computer science -- (IBM) removable disk pack

   disc: Agriculture (machinery)
         Botany (flowers)
         Recorded music (78 rpm, etc.)

•Sources: Webster's Ninth Dictionary, 1965; American Heritage Dictionary, 1990

But I am still curious:  which spelling was used in the old
advertisements for (1) music boxes, and (2) phonographs?

Robbie Rhodes