I want to know the answer to the following question:
What type of mandolin was becoming popular by 1930's?
All the background I'm provided with is that "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).". Is that enough to answer the question?
Answer: yes