I keep coming back to this circuit board for some reason...

This board consists of four panels - to get the rest of the board out of the way -

Panel A consists of the main receiver stage (processing a weak carrier signal into a TTL pulse stream), and three timers (deadman, outage < 2 minutes, and outage > 2 minutes).
Panel B consists of a +/- 170VDC supply, a circuit that converts a TTL pulse stream into a +/- 170VDC pulse stream, a resonant circuit that turns it into a sine wave, and a coupling circuit.
Panel C consists of the main logic, taking several inputs (some from Panel A) and converting it into data output (to panel B), and load control / rate LED displays (panel D).

Most of Panel D's function is already known - main +12/+5V supply, signal pickup, rate LEDs, load control relays, and input handlers (left floating or grounded).

The portion I'm not quite clear on is the U21 stage - the section where all the parts are marked 'not populated'.

Could use some thoughts from the brain trust as to what's going on there - component values are a 'best guess' as only a relative handful of units were produced with this stage populated.

CB2, PB6, PB7 are from the PIA on Panel C, Bit7 runs back to Panel C as well, where it is read as the top bit of one given address.
What I do know is that this section (and J3 on Panel C) are tied to an ADC daughterboard (based on National's ADC3711) - but without an actual unit at hand, all we can do is guess...

 

A section of a PCB
The portion I'm not quite clear on is the U21 stage - the section where all the parts are marked 'not populated'.
That means that the parts are not installed on the PCB, and so the circuit as shown is SIMPLY NOT THERE

 

...of course.

My question was what it was supposed to do otherwise in the one application where that stage is populated. Buffer, level shifter, ?? And why would there be a gate pointing right back into the data bus.

Oh well, thought I'd ask anyway.

 

In response to that last question: My question was what it was supposed to do otherwise in the one application where that stage is populated. Buffer, level shifter, ?? And why would there be a gate pointing right back into the data bus.

The answer is that some times a data bus is "bi-directional", meaning that data flows both ways. APPLICATIONS can vary quite a lot, some times.