From Computer History Wiki
The DD11-A backplane is a 4-slot UNIBUS backplane system unit; it provided 4 Small Peripheral Controller slots. It was normally installed in a BA11-C or BA11-E mounting box.
It was supplied with power, not by a power harness connected to the back of the backplane, as all later DD11 backplanes did, but through a paddle card which plugged into the 3A slot/connector; the paddle card was wired to a power harness which ran longitudinally within the BA11.
DD11-A's were mounted crosswise within the BA11, unlike all later DD11's, which were mounted longitudinally. This made them unable to hold hex cards, since such cards would have intruded into the space used to hold the cooling fans which ran down the wide of the BA11.