These are photos of the Atco as a ride-on. I have no idea if this attachment was an available extra when the mower was new or indeed if it was made by Atco. The castings and general construction are identical to the mower albeit without raised lettering on the side bars, only numbers. This  leads me to believe it is original although the fixing brackets are a bit of a bodge and definitely later. Whatever, the attachment will be restored along with the mower.