As an example 264 and 260 should be sortable for publisher Some records will have one, some will have both, it should eb configurable which to use and that we should not leave the sort field empty if one field is missing
In the field mappings of the search engine configuration there could be an integer option "sort fallback order". So, when indexing, the sort field is filled with the values corresponding to the mappings with "sort fallback order" = n for the smallest n where the corresponding fields are nonempty.