That's impressive dedication to fidelity...
Sorting the planking for the end compartments is easy because there are enough GWR Dean four wheelers to research, all conform to the pattern shown - which is not surprising given the turnunder at the bottom of the end. The real "dedication" comes with some of the other compartment partitions!
The GWR standardisation for the boarding was based upon:-
1/ coach ends are boarded horizontally;
2/ each internal partition is of two layers of boards with one layer "horizontal" and one layer "vertical";
3/ except for a partition with a sliding door when both sides of the partition are horizontal and separated by studding;
4/ opposite walls of a compartment are either both horizontal or both vertical.
So for a carriage with an even number of partitions, the boarding is arranged as:-
H + H ... V + V ... H + H ... H + V ... V + H where "..." is a compartment partiton.
and a good example of this is all-third to Diagram S9 as available from
Slaters. We have more fun when a coach has an odd number of partitions because in that case the standard breaks down. Discussion with the C&W gurus at Didcot indicates that coaches with an odd number of partitions probably contain 1st or 2nd class compartments where those compartments have either wood or Lincrusta lining above the garniture rail and hence the boarding is not as obvious to the occupants. When Peter does the 1st / 2nd composite to Diagram U4 - from Slaters - then we shall have to make a decision as to the truth or otherwise of "rule 5".