Did you notice the README.TXT file from that site where you got your ISO of Slackware64?
** WARNING! **
This is experimental current ISO builds. They are NOT official,
so use at your own risk!
If you have any comments/suggestions/error-reports regarding these ISOs,
let me know so I can improve the build-script! You'll find the script
New build every Tuesday at 0500 CEST
larstra (at) ifi uio no
Alien Bob has a mirror-slackware-current.sh script which will download
the Slackware64 sources from an official website to your computer, then
make either CDs and/or a DVD. That's what I did to get official Slackware64
-current. There are NO OFFICIAL ISO images for any Slackware* -current.
The ncurses pkg is on that mirror, so they just messed up building the ISO:
As for packages.slackware.it, read this thread