Fascinating, indeed...
I find somewhat hard to believe that the Castillo would have deaccessioned such a piece, but nothing is impossible.
Let's see... Bernat de Sarrià died in 1335, so the name and the date would go together as a supposed attribution that we do know... let's say "unlikely", to be polite.
The Castillo de Montjuich is now a (soon to be closed) military museum in Barcelona, but it was a military fortress with an active and bloody story up to the mid-19th c., when it started to be essentially used as a garrison, to be shortly after transformed into a military prison, a role that fulfilled until 1960, when it was donated to the City of Barcelona. It opened its doors as museum three years later, in 1963. The Estruch collection was auctioned in 1903, and acquired in almost its totality of 1300 pieces by the French collector Georges Pauilhac, who later on donated its own collection, including Estruch's, to the Musée de l'Armée, in Paris, when it can now be enjoyed by us all. So, I don't really think there's a connection between the Estruch collection and the Museum in the Castillo, although, again, it is not impossible.
My money goes, in short, also to the theory of a dealer trying to make a better sale by assigning a piece to an historical character and trying to substantiate such claim by pretending that it was in fact a museum from that character's place of origin who did in fact the assignation, "inventory number" included.
In any event, it could be interesting to contact the last director of the Military Museum in Castillo de Montjuich, Mr. J. L. Calvó,
HERE, he may know something about this labels, if there's indeed a certain number of them.