Francotte (imported by Von Lengerke & Detmold) and Greener Single Barrel Trap guns were introduced in 1895 I believe with solid ribs. Lefever introduced a SBT with a solid rib in 1905, but Baker was the first major U.S. maker to offer a ventilated rib on their SBT in 1909.

The Prechtel SBT first appeared at the Cleveland Cartridge & Target Co. tournament in Cleveland, Ohio June 1897



Hunter Arms introduced the Double Barrel Trap gun in 1920, and the 'Trap Package' included a flat beavertail forend (Schnabel forend available in 1922) with the reinforced barrel lug, Automatic Ejectors, Hunter One-Trigger, standard recoil pad, and two ivory sights.
The ventilated rib on the Double Barrel Trap was introduced in 1925.





Parker offered a Ventilated Rib on their doubles starting in 1923.
How about Baker and Lefever? Remington and Ithaca?