After some more searching I've uncovered loads more info... none looking promising for the Yokomo though...
It's part number is ZR-RTHB - in Japan it's known as a "trailer head".
This is where I found the part number
http://www.teamyokomo.com/japan/week..._070219_en.pdf
Which lead me to these...
http://www.modelsport.co.uk/index.php?product_id=22488
http://www.ajhobby.com/product.asp?pid=1242
http://www.rcfans.com/bbs/forum.php?...ead&tid=339380
http://www.teamyokomo.com/japan/prod...rth/index.html
Good luck, I think it will have to be via old Japanese stock or ebay and a complete bit of luck, I don't think you'll get one otherwise... Unless someone can make you one ?
It might even be worth posting on the truck forum ?