I've been looking for the same. I've confirmed that LanguageBreak doesn't work, despite what it says, on a lot of devices & firmwares that are older.
I tried it on PW3 on 15.3.7 before failing over a dozen times & going with WatchThis.
I am working on a Firmware Table & just added the details for the Jailbreak in the Jailbreak Table at
https://wiki.mobileread.com/wiki/Kindle_Firmware
If you find any information relevant to this let me know or add the information to the table