BA11-E Mounting Box
It was created for the expansion box role (hence the -E variant suffix) with the early PDP-11's (particularly the PDP-11/20). In that role, it was used to hold extra core main memory systems (each in their own custom system unit) and other devices, and had a blank front panel.
It came in two variants, the rack-mounted BA11-ES (including Tilt and Lock chassis slides), and the table-top BA11-EC. The rack-mounted ones were usually mounted in the H960 series of 19"-wide racks, which provided 63" of vertical mounting space, divided into units of 10-1/2". The 10-1/2" tall BA11-E mounting box was designed for these spaces
A BA11-E could hold up to six standard size (4-slot) system units; these were mounted transversely, facing down. They were generally mounted with the A-row connectors at the left-hand side.
A set of four cooling fans was also mounted at the left-hand side of the box, to provide air-flow to the boards and power supply. Since the fans intruded into the space 'above' the backplanes, the latter could not hold hex cards, only quads, plus a single extended length card.
Later versions of the H720 had a pair of connections to the standard DEC Remote Switching Control Bus, so it could turn other units off and on with its power switch.
The BA11-E's dimensions were 10-1/2" high, 19" wide, and 23" deep; it weighed roughly 100 lbs (including some backplanes and boards).