After some playing around I got this monster pushing 240hz as 1024x768 😲

I'm guessing you're just running it out of specs though.

Actually what manual says is that max vertical rate is 140Hz and max horizontal rate is 96 kHz.

So it just can't go over 85Hz at 1280x1024, and over 75 Hz at 1600x1200 resolution, because that would go over the 96 kHz hfreq limit.

Therefore, 1024x768 @ 100 Hz (81kHz hfreq, 113 MHz pixel clock) is well within specs.
Same goes for 2048x1536 @ 60 Hz (95.3 kHz hfreq, 267 MHz pixel clock).

These formats are not in the manual because they did not exist (weren't standardized by VESA) back in 1999 when manual was printed.

So the monitor has no trouble regarding syncing to these signals. The problem is more like the 2048x1536 signal has pixels smaller than dot spacing of the CRT so it may not be very useful.

OK, the 240Hz does keep me wondering, I don't know whether it just so large that the microprocessor in the monitor can't measure it and accidentally allows the picture to be displayed, or is it still within safe limits.

Don't know about 240hz. I know I can set it to 240hz and enable vsync and get 240fps on 3dmark. But anything past 120hz looks the same. Could just be my eyes or its down sampling or something.
It's a very flexible monitor.

Although it looks like crap at some resolution when the inter screen is filled. It gets fuzzy and distorted. Need a little work but I'll get it like new.