When Cowan Powers and his family recorded their old-time music from 1924-1926, his daughter Orpha Powers was one of the earliest known southern-music artists to record with the mandolin. By the 1930s, single mandolins were becoming more commonly used in southern string band music, most notably by brother duets such as the sedate Blue Sky Boys (Bill Bolick and Earl Bolick) and the more hard-driving Monroe Brothers (Bill Monroe and Charlie Monroe). However, the mandolin's modern popularity in country music can be directly traced to one man: Bill Monroe, the father of bluegrass music. After the Monroe Brothers broke up in 1939, Bill Monroe formed his own group, after a brief time called the Blue Grass Boys, and completed the transition of mandolin styles from a "parlor" sound typical of brother duets to the modern "bluegrass" style. He joined the Grand Ole Opry in 1939 and its powerful clear-channel broadcast signal on WSM-AM spread his style throughout the South, directly inspiring many musicians to take up the mandolin. Monroe famously played Gibson F-5 mandolin, signed and dated July 9, 1923, by Lloyd Loar, chief acoustic engineer at Gibson. The F-5 has since become the most imitated tonally and aesthetically by modern builders.
 What type of mandolin was becoming popular by 1935's?