Manuel Bouyer's suggestions for the 3Com USRobotics 3CP5609 PCI
16550-compatible modem card I have worked. It's up and running very
nicely. I sent him the diffs.

BTW, "3CP5609" finally now shows up in the search results at the
3com site, so it does appear that I picked the correct model number
from the board's collection of them...

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http://consumer.3com.com/modem/filelibrary/index.html

[...]
Windows 9x and Windows NT 4.0 Installation disk for the 3CP5609
(0005609-00) PCI Modem. . Place a blank formatted 1.44 floppy in
drive A: Double click the file in explorer and the file will self
extract to a 1.44 floppy in A:

3cp5609w.exe     file size: 221977 bytes     file date: 08-10-99 
[...]
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'Thanks' to everyone who offered assistance.