Those boxes are 4 speeds that replace 3rd gear with an over driven gear set. Then the 3-4 lever is flipped upside down so that when you push the lever and and get the handle in 3rd gear (on the H pattern) you are really putting the transmission into 4th gear, which is direct. When you pull the lever into 4th (on the H pattern) you are really putting the transmission back into 3rd gear which has been overdriven.
Those boxes will take more abuse than people think.