Whenever a Z39.50 search is performed on an ISBN field (only: all other search fields being empty), and the ISBN-10 field terminates with a 10th control digit "X", no results are found. Even though records are actually present, and are found if the search is repeated omitting the final digit (9-digits 10-ISBN field only). The correct 10-ISBN field is then found (complete with the final 10th digit "X") and can be imported into the Koha record. (Tested mainly with the opac.sbn.it:3950 Z39.50 server.)
The same does not apply to the LOC Z39.50 server: records with a 10-ISBN field terminating with "X" are found correctly.
Hi Giuseppe, I think this looks like a problem with this specific Z39.50 target server - did you try yaz testing it?