[desperate]

The boot environment is what runs from the EPROM on the mainboard (firmware). If, after pushing the power button, and watching for the power light to turn green, you hit the "escape" key, then the "5" key, the command prompt is listening forthwith (sometimes called the "PROM" but that is ambiguous). That is where "unsetenv monitor" is applicable.
The IRIX root prompt, however, is only available after the system boots, and stores its settings on the root partition, not in NVRAM on the mainboard. So using "setmon -x 60HZ" to change IRIX's graphics setting is not going to work unless IRIX has been installed to disk. It will not work if you have simply netbooted the kernel: there is no root partition.
You can, however, change the graphics setting for the current session by issuing the "setmon -n 60HZ" command to the root shell prompt. This should at least let you see if the graphics output is working with your display.

There's no hard drive in the Indy yet, I'm still just trying to get a working signal to the monitor, so any root shell commands will have to wait.

I was under the impression that "unsetenv monitor" would make the Command Monitor visible on the monitor, so I could continue from there and see output from bootp attempts. At least after an "exit" and then pressing "5" again, or perhaps an "init" in the Command Monitor would be enough to make the video output 60Hz instead of 76Hz. Apparently not.

Once I have working video output I'm going to connect an old SCSI drive I hope is till working, and a SCSI CD-ROM drive I haven't used in at least 20 years. But there's no point in connecting them now if I can't see anything on the monitor.

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I only know of one 13W3 to VGA cable on the market, the one with dip switches you can get through https://www.cablesonline.com/613mtohdmsvg.html, that works with SGI's. In the USA they also sell this cable through eBay and Amazon. You can check to see if you're in a similar situation. Otherwise the link above is for the original vendor. I have about six of these cables hopefully the never stop making them.

Every other SGI I've owned prior to this cable coming on the market used a separate small adapter and then a VGA cable. And back in my earlier days I only had one of these adapters so they were shared among all my computers.

So I'm going to conclude, in the absence of any further evidence, that what you have is a SUN Microsystems adapter cable.  And I say that because pretty much 99% of all the 13w3 cables I ever encountered were actually designed for SUN.

SUN had much larger market penetration than SGI ever did. By orders of magnitude higher. So it's not unusual that their cables became the dominant 13W3 cable in the wild. It's a shame that they're not wired exactly the same. I've seen some websites that rumor that you could break off a pin or two on a SUN cable and make it work with some SGI systems. But I have no direct knowledge of this.

I think (hope) I have an SGI cable, I purchased it through eBay from a guy who also had other SGI Indy cables for sale. He listed them specifically mentioning SGI Indy which is why I'm hoping they're wired correctly.

I intend to test the cable with my multimeter, but there are different pinouts described on different webpages so I'm unsure which one is correct, for example there's https://forums.irixnet.org/thread-382-po...l#pid18849 and https://wiki.preterhuman.net/13w3_to_VGA . Is there an authorative webpage for the correct 13W3-to-VGA pinout?

My multimeter's probes aren't thin enough to measure inner A1/A2/A3 so I can't check all pins without some extra thin pins (staples perhaps), but I'll also have to set up my vice to hold the cable connectors since my hands aren't steady enough to hold the cables and apply the probes to pins at the same time.