What you have is called a Dual High, Auxiliary Transmission, after the 3 speed, it takes special adapter to mount it on a Truck 4 Speed. Although it is actually a lower gear ratio, not Higher like a overdrive transmission. It was used in Model AA 131 inch wheelbase frames like Dump Trucks and Log Trucks, a nickname for it is a stump puller Transmission. With your Transmission in Low Gear, Dual High Engaged, Throttle down a little the Truck would crawl, you could walk faster that the truck. I've seen a truck pulling stumps with one, the front end comes right off the ground about 3-4 feet, and the weight of the Truck does pull out most Stumps.