Thank you chaley. A more thoroughly investigation of "content.opf" shows that
1) the original "#pub_series" does still exist
<meta name="calibre:user_metadata:#pub_series" content="{"rec_index": 27, "datatype":
2) a new entry has been added
<meta name="calibre:user_metadata:#pub_order" content="{"rec_index": 27, "column":
I believe you are right (the lookup name has been changed; and of course I also changed the column heading
) but I also think that while the original entry has been changed to a new entry the existing original entry has NOT been deleted. And this seems to be the problem (besides the problem that the pocketbook software uses a false series information).