The Kuhnke 653 and 657 series PLC's are older rack style systems, developed before decentralisation of I/O. Typical units comprised many I/O cards in slots, interconnected to power supplies, watchdogs, CPU's and memory devices, all housed in a single cabinet. Larger units could have up to 1000 I/O.
Although few of these PLC's are now built, a large number continue to operate in the field. Most replacement cards, with part numbers such as 657.440.02 are easily available.