Typical PO wagon in 1920?

[Penrhos1920]

What was the typical PO wagon in 1920 just a couple of years before the 1923 standard design? I’m wondering what was the ratio of 5, 6, 7 plank wagons?. Also what lengths?

I don’t expect that this is data is available for the South Wales valleys. But maybe for another area or the whole of the UK.

Modelling a representative coal train set in 1920 is difficult with only 4 different kits of end door wagons available in 4mm. So I’m thinking about what other design I ought to build.
Thanks