The high byte of fifth configuration register for the MBP_MSTR block is used to define which backplane slot position the Ethernet module resides in
the older Quantum controllers had a single slot Ethernet head module so the high byte of that register would be set to the slot number in which the NOE resides.

The new Quantum 140-CPU-6xx-xx controllers have a built-in Ethernet port and the controller itself is two slots wide.
The high byte is not set to either slot three nor slot four; rather a code of FE Hex is used to indicate the block be directed out of the native Ethernet port on the CPU.