Stumped.
I used gmaptool (run as admin) to split the gsupmap.img file,

Options
clicked on Advance Options
supplied the path to cGPSmapper
left Mapset Code Page alone = GIU language default

on the Split tab

Chose - Files for Mapsource
ck'd Compile Preview Map
Set the directory to the same as Mapsource
Provided a short Mapset Name
ck'd set mapset ID, ID to 702, set PID to1

Tried with and without specifying a file name

Split All - the process runs to completion without errors, generates a bunch of .img files including the required mapset.img file.
I run install.bat and it completes

Next on to Mapsource 6.16.3.0
this is run just from the mapsource.exe file and it opens just fine. There is no .dll file in the mapsource directory
When I ask it to open any file that resulted from using gpsmap, none of them are a valid mapsource file type.

other than the .img files
there are
,mdx ,.mp, .tdb files

I have repeated the operation numerous times, tried different directories but the result is the same. Obviously I've gone astray but I'm not sure what I'm missing. Maybe the .dll file I've read about.

Thanks for any illumination.