My guess was correct. The 13W3 is wired with separate sync. Here's the pinout -
2 - V
4 - Gnd
5 - H
This doesn't match any other vendor's pinout I'm aware of. A1, A2, and A3 (R, G, and B) seem to match the Sun layout. I am basing this on the (correct) blue bootloader screen that came up. The X desktop only came up with black, white, and grey, and I didn't do much to explore other than to verify things are more or less in order. I got lucky and the root password was blank. There seems to be some app software installed, so I will want to back that up sooner rather than later, and the software license files (if I can find them).
The power-on process seems to go like this:
1. keyboard beep (immediate)
2. RS232 handshake (DSR, RTS, CTS) lines asserted (you can see this with an LED test block on the serial port, they'll go from red to green)
3. keyboard beep
4. display test pattern (brief)
5. display clear
6. console displays self-test results
The second step should happen within a couple of seconds; the third after maybe 10 or 15 seconds. My system appears to be having a problem where it will only progress past step 1 if it's been sitting powered off for many hours. Now that I've got the display wired correctly it ran fine for ten or fifteen minutes before I shut it down, then failed to come up at all the second time. It was doing this before I had the display wired correctly (made it difficult to troubleshoot!) but I didn't know if something was happening on the display that I just couldn't see, or what. As it turns out it's like the CPU never exits reset; it does the exact same thing if the processor and memory modules are removed. The CPU also gets very, very hot when this happens - much hotter than it does when the system runs normally. I have no idea what the problem might be. If it gets past step three, it seems to run fine, possibly for as long as I like. But if I shut it off, however briefly, it needs to sit for >6 hours before it will self-test again.
Also - the second machine arrived this afternoon. It seems to be an InterPro 2020 (C300), and I will need to find a source for the DB5W5 cable it needs to attach a monitor. Looks like it takes 30 pin SIMMs. Good thing SAQ left me with a pile of them. Possibly I could use some more...