I hope he won't mind me quoting him here but the font of all knowledge on these matters, Metad, recently advised on a similar thread in a parallel forumverse somewhere that...
"Z stands for the world wide version, the same as X-version, before 1980.
Units with Z are multivoltage, and manuals for them in various languages.
As far as I know, Sharp started to mark boomboxes with Z in 1980. GF-666Z was the first. Funny enough, GF-555 made in 1980 marked with X, but GF-666 of 1980, already with Z."
I hope this helps you with the facts here - thanks to Metad for this very informative detail
