To me, looks like an old belly mower converted to a 3pt. It didn't come with a gear box, and the 'inventor' made do with what he could dream up. It now is what we call a 3pt. finish mower, of sorts. Big pulley would give the mower blades lots of spin, and was probably the intention, as the PTO spins at 540 rpm, and mid mounts usually spin at 2000 rpm. Just a guess on my part though.