They are mainFRAMEs - Each rack (like) unit had two frames installed on each side, which could be swung out like a door (after opening the doors), Depending on the machine and usage, this frame was a single component - or in case of CPUs maybe only a part thereof. For more compact items, like memory, usually a half-frame featured one compact component. For maintenance both sides had to be accessible, that's why you'll (almost) never see a mainframe pushed against a wall, but with at least 95 cm walkway between units (*1). The closest to compare may be two 19" racks pushed back to back.