I've heard very briefly of these. If I remember correctly: this mode came from pixel-doubling the output of the Apple IIe LC-PDS card. I haven't heard of any other tools that you can use to get to it, but most displays that the LC family supports allow it in one way or another. So, most of Apple's own fixed-sync displays at least up to the 14" ones support it. The 16" one probably does but on a Mac you'd use that at the higher resolution of 832x624 most of the time. Larger displays weren't initially supported by the LC and LCII so I doubt that Apple bothered to make those displays compatible with this mode, or tested it extensively if it does "happen" to work.
I can't speculate as to why it was built into NuBus video cards except perhaps that someone internally wanted to test it to get more out of a hardware budget or that Apple had been throwing around the idea of building a NuBus IIe card or similar.
In newer times, I used a tool called SwitchRes to work around my fixed-mode adapter's configurations on newer multisync machines, such as the 7300 (but really: almost any machine newer than Quadra 700/900 in 1990 is likely to have some additional pixel functionalities) and also to work around the "default" list of resolutions on my blue-and-white and the monitors I was using. It let you select from a list of common resolutions if you had a fixed-sync adapter and it let you pick from a fairly long list of suggestions of modes. I don't know if they were pre-determined or if they were calculated or guessed.
The weird part is that after so many years only one single reference lists the correct speed. Usually in the intervening years someone's emailed someone at everymac/LEM or another of the million different mac spec dexes and gotten them to update their listing for the machine's speed.
My initial thought was that it might have been a system whose motherboard was swapped, perhaps for a board such as the 5260. OS compatibility is usually "fine" when in close proximity or using a certain model's install disk on an older system, and you might never notice except if you look at System Profiler, which might list the exact configuration it thinks you have.
Just casually looking at the service manual at http://tim.id.au/laptops/apple/legacy/powermac.perf_5200.5300.pdf Apple lists 120MHz in there too.
Granted, given that these are almost certainly written in English in the USA and then (if I had to guess: not localized into other languages) distributed, the person who wrote this likely did not have a very good opportunity to review an actual Performa 5320.
The box is definitely the wildcard, and it is the main bus in my personal "what if someone swapped the board at some point?" theory.
Those sound cool. I've never heard of them.
I know people always talk about Macintosh Garden, but there're a lot of shareware and freeware titles that don't make it on there, like these two Monitor apps. My "The Art of Darkness" book mentions 4 or 5 freeware and shareware After Dark modules that are now unobtanium. All the original FTPs & websites have gone dark, all the commercial sites only ever hotlinked, and no one bothered to back them up.
Is it one or both of the retaining clips at the top of the door, the clips at the bottom of the door, or the door itself? It's incredibly unlikely that you'll find a TAM drive door popping up for sale on its own but if its one of the clips that broken and you still have the pieces you might be able to glue everything back together.