William (Giullermo) Del Pilar was a New York classical guitar maker born around 1921. Del Pilar came from a Puerto Rican family of cabinet makers, and got his start in luthiery when he was asked to fix a guitar broken in a fight. Soon he was making guitars and selling them for $10 a time. After marrying Ana Elisa Ruiz in the early 1940s Del Pilar moved to Brooklyn, setting up shop at 396 Atlantic Avenue. Around 1945 his son, William/Giullermo Junior, was born and William Junior later joined his father in the business. They later moved to 220 Atlantic Avenue, when working together they produced around 40 guitars per year at most. Del Pilar Senior gained recognition in 1959 by carrying out emergency repairs on Andres Segovia's guitar before a New York concert.  By the 1993 Del Pilar senior had retired leaving William Junior to carry on the business.

Source; Master Guitar Makers Are Leery of Their Art. NY Times. February 3, 1993