This is a body only model in 4mm:1ft (OO) of the Cambrian steel-built mineral wagons of 13T capacity - forerunners of the slightly taller 16T mineral wagons that followed in huge numbers. This particular model is a simple rescale of my 3mm model of the same thing, itself rescaled from 2mm, but I have removed the interior strengtheners at this scale because they shouldn't be needed, and would have needed to be made thicker to allow it to be printed in the more economical plastics.
PS, added 2023-11-22:
Many thanks to tigger 1805 who requested this rescaled design, for sending his photos of the unpainted but basically finished first ever print of this model.
See on RMWeb (21D)
https://www.rmweb.co.uk/topic/182811-3d-printed-bodies-of-early-all-steel-mineral-wagons/#comment-5344253
I'm not sure if leaving out the internal strengtheners on this design was the right thing to do. Let me know if you have a strong opinion on me putting them back in.