From the Gibson Website in the History section:
1903-World War I Gibson dominates the mandolin world in the golden age of the mandolin orchestra. Refinements such as a smaller size, rounded back and elevated pickguard, combined with aggressive marketing, make Gibson the leading mandolin maker. Gibson bypasses the conventional retail network by enlisting music teachers as "teacher agents." Gibson encourages and supports teacher-agents in forming mandolin orchestras and features photos of ensembles in ads and catalogs over the caption "Every One a Gibson-ite."
1921 Gibson employee Ted McHugh, a woodworker who had sung in a group with Orville Gibson, invents two of the most important innovations in guitar history: the adjustable truss rod and the height-adjustable bridge. All Gibson instruments are still equipped with McHugh’s truss rod, and traditional jazz guitars still utilize the bridge he designed.
1922 Gibson introduces the F-5 mandolin and L-5 guitar. World War I killed off the mandolin orchestra and given rise to the tenor banjo, threatening Gibson’s existence as a mandolin maker. In an effort to revive the mandolin, Gibson acoustic engineer Lloyd Loar designs the ultimate mandolin, the F-5. Its new features include f-holes, a longer neck and hand-tuned top, tone bars and f-holes. As a companion member of the Style 5 family, he designs the L-5 guitar. Loar and his creations will become legendary, but Gibson almost goes bankrupt, and Loar resigns late in 1924.