Fragments found a while back have been identified as being part of a shield - 5th century BC I think.
http://lamarinaplaza.com/2014/04/05/descubren-parte-de-un-tachon-en-unos-restos-de-bronce-localizados-en-una-tumba-ibera-en-2001/
and less complete in English
https://www.euroweeklynews.com/news/spanish-news/item/119801
Thanks for finding that, Peter. Neither site sees to mention the diameter - that's a shame.