I've seen wagon labels before, sold fresh as daisies on railwayana stands. I've seen the clips on the side of wagons they fit in to, but never the two together. Until last week at Quorn on the GCR.
This would have been common enough in the days of 4 wheel wagon freight trains but I don't really remember these and I suspect neither will many readers.
Modelling in 4mm scale would just be a splodge of paint, something I'll be doing more of in future. Colour is a pale fawn rather than white. Finescale nerds will shrink real labels down and then criticise each other if the destination is wrong...