Probably a reverse lock-out. I've seen them on early '40's models,as well. That's where the 2-speed cable attaches,on T.F.,and later trucks. P.T.O. levers,of that era,were usually floor-mounted,due to the linkage used. Cable-actuated units,(dash-mounted), didn't become popular,until the'60's. Try using reverse without using the lever. If it won't engage,then you'll know for sure.