Because there's never mentioned the additional detailed build number with these initial fw builds, I, on a first glance, assumed that device being a test/evaluation Kindle, and therefore hoped that I could skip the downgrade.
So, as a first step, I tried to apply the ;installHtml search command right out of the box. Didn't work. But the normal procedure did.
Unfortunately, I've thrown away the sheet of paper where I noticed the two "5.8.0" build numbers. But I'm absolutely sure that the initial factory download linked here in posting #1 had a lower build ID than mine. Though both displayed as "5.8.0"! There even might be more than these two 5.8.0 builds for the KT3...
Gesendet von meinem Google Pixel mit Yabba-Dabba-Talk.