[un_pogaz]

Okay, I'm posting here, because it concerns the very fresh development of Calibre, and I'm not sure if it's a bug (or at least, I lack explanation)

for a plugin I'm looking at the FieldMetadata from "calibre.library.field_metadata", and I discovered that the "custom_series_index" fields are not marked as custom fields.

{
    'table': None,
    'column': None,
    'datatype': 'float',
    'is_multiple': {},
    'kind': 'field',
    'name': '',
    'search_terms': ['#custom_series_index'],
    'label': 'custom_series_index',
    'colnum': None,
    'display': {},
    'is_custom': False,
    'is_category': False,
    'link_column': None,
    'category_sort': None,
    'is_editable': False,
    'is_csp': False,
    'rec_index': 37
}

If it's a bug, it's old, V4. It would be too weird that it went unnoticed all this time.