system
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Thanks to posters xtinguish and NON (above). 
What do you know, it did re-install on re-boot!
I didn’t immediately see info on the forum link from xtinguish, but there are now a bazillion pages and growing on that topic, so… Anyone following NON’s suggestion may want to note that the system file checker he refers to requires an installation disk (presumably an OEM rescue facility would also work, if you can find it).
However, fingers crossed for everybody to restore sfloppy.sys it through a straightforward re-boot!