Linguistic approach:
In the Spanish Foro por la memoria we find different ways how the Brigada XV is named:
- ... en la decimo quinta Brigada Internacional ... and: Viva la Quince Brigada!
One would not expect quince brigada, but correctly the ordinal number decimo quinta brigada.
- XV Brigada [often], Brigada XV [once]
Is the XV ordinal number of the counted brigades, or a cardinal number as the brigade's name, and brigada the apposition?
Not even the Spanish in their home language do it always the same way.

From the history of battles we can say: The brigade in question must be the 15th.

[the accent over the e in decimo omitted due to HTML difficulties]