Kindle for PC 1.17 was the last version that kept all the books in one MASS folder before splitting each book into it's own folder. It's also the last version before they changed the azw format slightly so that Calibre no longer recognized it.
I suspect that it may be another major breakpoint. Amazon has forced upgrades before (on KPC and kindles) so that you had to be at a minimum level for the thing to even work!
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There is no such thing as a stupid question!
Corollary -- so long as you read before you post and stay on topic. :)
filehippo.com is a good and trusted file archive site that has been around for 14 years. They have archives of all of the Kindle for PC releases if you ever need an older one and can't find the old amazon download link. Of course, while they are not a shady site, like many free sites run by small groups they rely heavily on ads... so their pages have a lot of ads on them. If you use the site keep an ad blocker running and be careful to only click on the right download link.