As a D flip flop has tow outputs and the second one is the first complimentary, the addition of an or gate allows to address the lower, unbanked 16k.
So if a '138 is fed with "-SysRegion" select signal...
Four controlled 2k regions can be selected independently: three ROM select lines and a RAM select line. Later I'll deal of how this is tied against SRAM and ROM, but the following is the goal to achieve.
There's overlap on the MOS memory and also on the ROM region. That's more or less what's wanted: overlap must be controlled, mainly by switches. Most people (including me) doesn't have material to read a TMS27xx, even less writing it. By using an standard 27c64 or 2864 (I think it was this reference) and three select lines, ROM can be selected in 2k chunks, allowing both partial (if > 0 but < 3 system roms failing) and full ROM replacement.