You wouldn't believe it, but I've looked all day for that code - RMEY. I found it about ten seconds after I posted the query!!
It wasn't always easy to find the code on the wagon....
The first one had canvas covers with the code painted on the canvas, and the paint promptly fell off leaving a sort of ghost of the number and code, RMEY 2417 if I recall correctly.
These were not new but were articulated rebuilds of existing wagons, although I'm not sure if they were already triple deck or the top deck was added in the rebuild. I think the pair took one of the existing numbers of the two wagons articulated together.
There were considerable variations in appearance, not only as built but as modified in service.
As I said, the first one was roofed and enclosed with a green canvas material.
At least one more was completely enclosed in pre coloured steel ribbed panelling in blue and white with alternate panels in each colour. As often was realised after this sort of conversion occurred, the top deck cladding was replaced by mesh to allow ventilation of petrol fumes from the motor vehicles.
I saw a pair run through Midland on a PN freight to Kewdale in 2004 (or was it 2006?) while I was photographing the Spirit of the West emerging from its shed. One was as I described, with what appeared to be one of the "fully clad" versions modified with mesh on the top deck and another that had a much flatter roof than the former fully clad wagon, which suggested to me that it had been (re-)built new with the mesh top deck.