Thanks Manichean for your response, and the great link to the wiki entry.
I was working from the online
Calibre User Manual page for HTML to MOBI conversion, which has both --series and --series-index
But I agree with you on the wiki entry, I cannot see any mention of book series in the
EXTH record types. However, there are a lot of unknowns in there and this wiki is unofficial. Perhaps a later version of the format will have the series.
So perhaps, for now, the Calibre User manual needs to be adjusted for Conversion to MOBI to exclude the mention of --series.