UPDATE to my previous message
I found KoboTouchExtended.zip version 3.4.4 earlier in this thread and have successfully installed it in Calibre 4.23. Woohoo! Is there a more-recent version that works with Calibre 4.23?
I've also installed KePub Output.zip version 2.83, which I found in the thread for that plugin. It's working, but I'm wondering if that is the most-recent version that works with Calibre 4.23?
Many thanks!