This kit is the BR SB Flatrol 20 Ton Trolley and is the same model as the preserved wagon at the Bluebell Railway. 
 
The pack comes as two variations:
Just the kit with 3D printed parts where  wheels and couplings need to be supplied separately. NEM coupling socket is modelled onto the chassis for easy attachment.
 
With 2 wheel axles and 2 long NEM couplings to make a full wagon. This variation is subject to stock levels and available supply.