It seems PR motors made a gearbox for dumpers in 1955 known as the cm, constant mesh. I will have a look inside this one as the older crash boxes are described as fabricated and cast steel. My one is a cast alloy, model not on the plate! It still seems to engage and drive with the diff working fine, a test would be under load. Next to find how it originally looked, the late 90 yr old owner who gave me it cannot remember, he towed it around behind a grey fergie! Cheers the noo.