So this is the best I can do for now. It's not remapping because it starts in the bootloader before the MMU is even enabled. My only guess is that the graphics board on the PC-X is different from the one in the PC-D. If you look at the pictures that rfka01 posted of the PC-X the graphics board has an 8031 which the PC-D has an 8047. It also has an extra ROM chip. Maybe the 8031 presents a terminal like interface and can write directly to the VRAM and the extra ROM is a character ROM instead of RAM (the tech manual says this is an option).