Thanks everyone for a very informative discussion - I find this both interesting and very helpful.
If I may ask something else ... were PO wagons generally of the same design just different sizes (like 3, 5, 7, etc). For example if a company had some 5-plank wagons would they be the same as another company's 5-plank wagons or would they be different in design? The reason I ask this is I can imagine that the likes of Hornby, Bachmann and Dapol will not cover all (probably most) of the POW that were out there by company, but I see that some people create decals or stickers of other POW that may not have been produced by the Model manufacturers mentioned .... would it therefore be "historiacally accurate" to transfer these decals to the correct sized wagon (eg. a 5-plank)?