I believe N were rather more specifically loco coal wagons, the Great Western not going in for building its own general mineral wagons, relying on the coal &c trade to provide their own or else hiring them in from the wagon companies.

Crocodile is, I think, a telegraphic code - so I wonder if it was chosen on account of the diagram letter C rather than the other way round. C covers trolleys, not all trolleys were crocodiles.

So, I think that apart from O for open and V for van, and the idea that specialised vans come after V in the alphabet, you're looking for more system than there really was.

Other railways got by with just D for diagram!

There's a bit more logic or at least system on the carriage side, with A for bogie first, B for bogie first brake (were there any?), C for bogie third, D for bogie third brake, E for bogie composite - except that where F ought to be bogie composite brake, such carriages seem to have been given E diagrams...

And then there's the auto trailers with an entire alphabet to themselves...

I have a theory that there's a hidden code in the GWR diagram soup. If you arrange them in the right way, they will reveal the year wagons changed from red to grey 

1 minute ago, JimC said:

Don't forget the "Felix Pole" mineral wagons.

True, I had, as they're well after my period!

I was just looking at the buffers used on the Siphon, can I ask what make those are? They're just what I need for another project, I think.