From audiobookshelf:
series and series-part aren't standard tags. Currently, there is no way to write these tags directly with Opus.
Mp3tag and foobar2000 can create and display them.
FFmpeg can write them:
@nodeselect
@nofilenamequoting
ffmpeg.exe -i "{filepath}" -metadata series="Hello World 1" -metadata series-part="Hello World 2" -codec copy "{filepath}.mp3"
ExifTool can read them.