Thanks, Kermit2 - I realized that in my state of exhaustion yesterday evening (lots of yard work in humid 100°F weather) that I'd transposed the 1N4372A to 1N4732A; the latter of which is in fact a 4.7v 1 Watt Zener.
That DOES change the output voltage, but I'll need to throw a model for that Zener into my simulator to make sure.
[eta]
Couldn't find a model for that particular diode in a short period of time; but the output should now be approximately 25v, give or take some, depending on the load.
The output load is a 26vdc relay that is PCB mounted. I don't know the current requirements of the relay.
I guess that when the signal at power source #2 is high, the output of the circuit is capable of energizing the relay, when the signal at power source #2 goes low, the relay is de-energized.
I'm seeing about 24.5v with that much of a load on it, and it takes an extra second for the voltage to reach that point.
Note that there is no reverse-EMF suppression diode in these relays. This circuit really needs a reverse-EMF diode added, or Q6 is going to be blasted.
What's this circuit in? A vintage military aircraft? We used extremely similar relays in F-4 Phantoms.