> Why is Alcon the parent rom ? When Slap Fight was released in july '86 & Alcon later
> december '86. I thought the parent roms were the 1s released first?
because Slap Fight has a 'for use in Japan Only' message, while Alcon does not. The home ports might have been called Slap Fight, and many of the bootlegs / unlicensed cabs might have also carried that name but there's no real evidence that Taito officially used that name outside of Japan. (I suspect the home ports ended up being Slap Fight either because they were based on the bootleg, or proto/location tests before the game had been localized, much like how LED Storm is based on the early / 'proto' version of that game)
Latest World / Asia / Us release takes priority over a Japan release as long as the original copyright holder is still displayed (in this case Taito, even if it was developed by Toaplan, their name isn't display on any set)
It is rare for a Japan version to be a parent and no reason to make an exception here. One of the cases where that has been done is Puckman, because Pacman wasn't a name used by Namco, only by Midway at the time.