Wheeeeeee !
Webscriptions support made the changes to the IIS tables and I was able to do successful PRC downloads using the Kindle browser
half an hour since, that's _1632_ and _The Notebooks of Lazarus Long_
http://www.webscription.net/ps-379-40-1632.aspx
A big Thank-You to Becky of principledtechnologies.com (Webscriptions support) and to everyone in this thread who helped identify the problem and its solution.