Thanks again, I read the perl code in the MobiPerl program file
exth.pm, as referenced in
https://dev.mobileread.com/trac/mobiperl without finding any "series" information, so I have come to the tentative conclusion that it is not there in the mobi format.
I have written a
Groovy program to clean up the html of my books and call the various calibre command line programs, so in the future I can easily come back to this problem and automatically read the series information I have stored in the mobi comment field and save it somewhere else, or use it in the book filename for sorting.