Mounting box
From Computer History Wiki
In older computers, a mounting box (sometimes called an expansion box) is a metal enclosure (often sized to fit into a standard rack, such as a 19" rack) which is designed to hold a number of the printed circuit boards which are part of a computer - either the CPU, main memory, or peripherals.
The mounting box often includes a backplane, for the boards to plug into, and may also include a power supply, although those are often mounted externally.
