:In traditional Basque usage, Guipuscoa was the provincia: Check Philotea from 1821: 
 ::Topatuco dute bear bada Provinciaco eusquerarequin batian Nafarroacoa, Vizcayacoa ta Arabacoa, 
 ::"They will find maybe along the Basque language of the Province [i.e. Guipuscoa] that of Navarre, Biscay and Alava". 
 :However, as you say, the Ley de Territorios Históricos created this new category, for reasons unknown to me. 
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