It is an option product compatible with HHKB Professional series (PD - KB 400 W, PD - KB 400 B, PD - KB 400 WN, PD - KB 400 BN, PD - KB 420 W, PD - KB 420 B, PD - KB 400 WS, PD - KB 400 WNS, PD - KB 420 WS).
eBay