Many years ago my father hauled his bulldozer on a lowboy and the old tractor had a Brownlite or "Brownie" transmission in it but both units had a regular shift lever. I wonder if this is a gearbox with just a high-low range since it just has the one lever. I have seen a second transmission installed backward to give much higher gearing. An example is if second gear was say 3:1 then the input shaft has to turn 3 times to turn the output shaft 1 time but backward turning what had been the output shaft once would then turn the former input shaft (now output) three times. Its been 60 years but I think they used a transmission with spur gears instead of helical gears for the one in backward. My old flatbelt Monarch lathe uses this principle for high and low gearing.

Evan