I have a pulley simular to this except it has six splines just like the 44s. I always thought it was for some sort of an implement like a blower or feed grinder, but why would you want to slow them down???
I think you should get it rebuilt and put it on a 55 or 555 as the belt speed would be increased quite a bit. This would make a good blower tractor as you now fill the taller 70-80 foot silos with ease.