There are a couple more similarities to Z-out & R-type than you might think.
Z-out is a follow-up to X-out,another horizontally scrolling shoot-em-up.
And R-type,X-out & Z-out were all released by Rainbow Arts.
The irony about the R-type conversion,was that Rainbow Arts were threatened by a law-suit from Activision regarding another shoot-em-up they eventually released & renamed as Denaris.
(Programmed by Manfred Trenz for the C64,who went on to do the Turrican games).