This is a Conformally Coated I/O Module.
The Parallel I/O Module (PIOM) provides the link to the processor rack and to other I/O racks in a parallel I/O configuration. The PIOM decodes I/O module address data received from the CPM and generates card selects to the appropriate I/O cards in its rack. Slot N of the I/O full rack or slot H of the I/O half rack houses the PIOM.