Thank you Adam. They are, from left to right;
Smith Hood & Co (Dundee) [coal merchants] 6 plank steel underframe,
Cox Brothers Ltd (Lochee) [Jute Mill] 5 plank, wooden underframe, no side door,
JT Inglis & Sons (Dundee) [coal merchants] 5 plank, wooden underframe, with sprung top door.
All are RY Pickering products and registered with the Caledonian Railway.
The central vehicle, which you rightly identify has no side door. This would be because Cox Brothers had a dedicated coal handling facility at their Camperdown Mill, at one point, the wolds largest jute mill, which had its own branch line and at one time employed thousands mostly female workers. Large parts of the mill remain, although now converted to residential or retail uses. The mill had 57 boilers and were served by a 282' brick chimney known as Cox's stack. It remains a significant Dundee landmark. Hence the need for vast quantities of coal.
John