MX15-B Memory Multiplexer
Some DMA devices on the UNIBUS (such as the RK11-E) were able to do 18-bit transfers over it; they used the two UNIBUS parity lines for the two extra data bits of the 18-bit PDP-15. The two extra bits were not used by the PDP-11 or its memory, but went straight through the MX15-B, directly to the PDP-15's memory.
Memory addresses in the PDP-11 (which uses byte addressing) were converted to PDP-15 word addressing by dropping the low bits, and conversely for transfers in the other direction. It also converted DATIP and DATOB type UNIBUS cycles (which have no exact equivalents in the PDP-15's memory system) to PDP-15 memory operations with the same effect.