Looking for information on the Espinoza Family

[Luis Espinoza]

My name is Luis Espinoza

My father departed Blue Fields on 20 April 1920. The Manifest of the SS Manauga has him listed as Carlos A. Espinoza B. He was 18 years and 8 months at the time he left. He could speak both Spanish and English. Under where born, he listed Castillo Viejo as the Town/City where he was born.

I'm trying to find out:

1) Where Castillo Viejo is in Nicaragua?
2) What is the significance of the "B" after his last name?
3) Who were his parents or other relatives?

I have never known the names of my grandparents on his side of my family.

I'm sorry that I do not speak Spanish so this correspondence has to be in English.
I would greatly appreciate any help I can get in my quest to identify my roots in Nicaragua.