After I got my old //c working again I went through the box of floppies I had on hand. These were all from the 80s and with a few exceptions were all non-commerical disks that we'd copied from friends or as backups. I set about working backward and bootstrapping from a "known-good" state via ADTPro and set to capturing as many of these disks as I could as archival images.
I was surprised that almost all the Apple disks I went through still worked after 30 years of suboptimal attic storage (the same cannot be said of all the DOS disks I have, some of which have melted into their paper sleeves) and I was able to not only archive them but start booting them and using them.
There was a series of disks that I did not remember, all of the same brand and with precise typewritten labels. These were utility disks, mostly, probably given to us by a friend: Appleworks, FileCopy, a couple of sides worth of mystery meat ("Optimization Program"? What's that?).
These disks did not boot. I sat them aside to consider later on. Well, now it's later on and as it turns out the ones that don't boot all have the same weird version of ProDOS on them and fail in the same way, on hardware and in emulator:
A weird version of ProDOS? Where did it come from, how did it end up on my floppy disks? The earliest 2.0 version of ProDOS I've found in the usual places is 2.0 from January 1992. Is this a pre-release build that made it into the wild? How did this guy, whomever he was that gave us these disks, get it? Why was it even given to us -- our family had moved on to a 386 MS-DOS machine by late '91, I can't imagine anyone soliciting Apple programs at that point. Regardless, if it doesn't work in 2019, it can't have worked in 1991, which means someone gave us these disks that, as far as we knew, didn't even work. Wow, thanks.
I don't know any of these answers. I made copies and replaced the ProDOS with a different version and the disks fired right up ("Optimization Program" ended up being Disk Disintegrator Deluxe 5.0).
Anyone know this version of ProDOS and why it would fail to boot? I don't expect answers to the rhetorical questions.