Interested generally in Steam-wagons I wondered the basis of the rather attractive Moss-designed, 1921 model wagon in the latest 'For Sale' ads.
It is most likely a freelance, but the long, low side-sheets (normally the bunker sides) and more so that distinctive drop-step footplate, rang enough bells to send me to my copy of Maurice Kelly's The Overtype Steam Wagon to search for its possible inspiration.
Result?
No close matches but ideas from Allchin, maybe Tasker – but there were a goodly number of other, smaller manufacturers about.
So if's still possible it was based on a particular make.