The 584C (and earlier versions) is not a KSU, but can become one when combined with a power supply and punchdown blocks. It's a "card panel" or "card strip" or "card cage" which can hold up to 13 1A2 line cards, and many panels can be used together to build a huge key system. It is desigend to be mounted in a standard 23-inch-wide telecom rack, or mounting bracket. Connections are made via 25-pair Amphenol-type cables. An interrupter is mounted on one end to control the switching on and off of lights and bells.
It was used in the 512KSU.