My first post here! This is a DEC LK-521 prototype keyboard.
It is similar to the DEC LK-421 keyboard used with their Unix terminals/workstations except it has a peripheral bus (like Apple's ADB) instead of the mouse and keyboard connecting to the computer with separate connectors. As far as I know, the bus was never used in production systems.
I think that this keyboard, along with the Sun Type 3, was the inspiration for the layout of the HHKB.
