To make sure that the new keystore is installed, I redid the jailbreak following the thread here:
https://www.mobileread.com/forums/sh...d.php?t=186645
In the step where we extract the contents of kindle-5.4-jailbreak.zip to the root directory of the Kindle, I replaced the extracted keystore with the updated one that you linked to above. I'm still getting the message "This item is not signed by an authorized developer."