I went into the .aff and deleted the "." from the WORDCHARS list as per
this thread.
Presumably, this is the best of both worlds: keep whatever incremental sophistication has been added to the newer dictionary and take out the annoying "period" thing.
Good to know wbout how Sigil engages with the preferential dictionary folders. I was trying to figure it out from the preferences screen but not getting it!
We're getting there...