[jurialmunkey]

Just an FYI - plugin://plugin.video.themoviedb.helper?info=similar&type=tv&imdb_id=$INFO[ListItem.IMDBNumber] doesn't work and I have to use plugin://plugin.video.themoviedb.helper?info=similar&type=tv&query=$INFO[ListItem.TVShowTitle] instead.

When looking further into the problem it seems TV shows don't have the tt prefix attached to ListItem.IMBDNumber - instead they either have no prefix or have xx instead. Episodes seem to have no prefix at all.

EDIT: Using the TMDB TV scraper.

If it doesn't have 'tt' at the start, then it's not an IMDb ID.
Some of the tvshow scraper addons misuse the IMDbNumber field and put TMDb or TVDb ID in it instead.

It's annoying as there is nothing to indicate what "type" of ID it is - otherwise you could pass it to the appropriate param in the plugin as it will also take &tmdb_id= or &tvdb_id=

Nothing I can do from my end of things other than suggest that the scrapers stop misusing the IMDbNumber field.

The 'xx' is a Kodi bug inside info dialog for addons. See thread here:
https://forum.kodi.tv/showthread.php?tid=346179