I've got an HHKB Type-S that was manufactured in January of this year, and it's key travel is noticeably less than my purple slider RealForce 104U. So for full key travel with a stock silenced board, purple sliders are a must. If there is a special black type-s slider, I don't know why it wasn't made to provide the full 4.0 mm stroke. Why else would they bother making a different version if not for keeping the standard 4.0 mm stroke?