The DD11-P backplane is a 9-slot MUD backplane system unit, with special modifications so it could be used to hold the KD11-E or KD11-EA CPU of the PDP-11/34 or PDP-11/34A (respectively). The CPU, which consisted of two hex boards, was installed in slots 1 and 2, which were specially wired to hold them.
(The details of the modifications are not yet worked out, but could not have been extensive, in view of the above. Certainly the bus grant lines would have been wired past slot 2, and there may have been an inter-card bus of some sort.)
It was normally installed in a BA11 mounting box. There are two versions, the DD11-PF and DD11-PK, intended for use in the BA11-F Mounting Box and BA11-K mounting box, respectively; they differ basically only in the length of the power harness, although the two provide +5VB (battery backup) on different pins of the DEC power distribution connectors.