I'm having the same problem. Updated from 3.3 to 3.4 without an issue. But I can't get it to update to 3.4.2.
EDIT: I figured out the problem. It's Amazon's fault. The file they show for updating both 3.4 and 3.4.1 was designed for 3.4.1. to 3.4.2. (That's why its name is Update_kindle_3.4.1_3.4.2...) I went to Archive.org and found an older Kindle 3 update page (in the Wayback Machine) and in it I was able to download the 3.4 to 3.4.2 file (Update_kindle_3.4_3.4.2...). My Kindle took to it right away and is now registered.
I've got the three files downloaded now, so will be willing to send them. The way I found them was by going to Archive.org and placing this link in the Wayback Machine entry.
https://www.amazon.com/gp/help/custo...deId=200529700
Then I chose May 18, 2017 and found the older download page with the missing downloads. (I was able to replicate the search.)