Hi Elena,
In answer to your question, I am a descendant of the Inzunza family that lived in Mocorito, Sinaloa, from the 17th century onwards. The family is mentioned in books like Eustaquio Buelna's Apuntes sobre la Historia de Sinaloa (circa 1885), Jose Ramon Velazquez' Apuntes de Mocorito (1994) and Gustavo Aguilar Aguilar and Wilfredo Ibarra Escobar's El Origen de la Banca en Sinaloa (1998). The last tome mentions at least one Esquerra (with an a). My grandfather Pedro Inzunza Gaxiola had a business relationship with an Esquer in Sinaloa at the end of the 19th century. My feeling is that the Ezquerro, Esquerra and Esquer families may all be branches of the same Spanish settlers in Sinaloa -- I have seen documents where in fact the same gentleman will be referenced in the text of the document with a "z" and yet he signs his name with a definitive "s".