R131 Wagon with wheel load

While I'm not the toy train collector I once was, there are still a few odd models that have lodged in my brain. I remember coveting this, R131 Flat Wagon with detachable wheel load as a youngster. 

Looking at the Hornby Collectors Guide entry, it appeared when I was four, so not too many years before I took ad interest in model railways. By that point, a staggering 41,000 had been made. You don't see production runs for a single wagon in those sort of numbers now!

I suspect that the idea of getting some "free" wheels appealed to me. I certainly wasn't looking at prototype fidelity as I doubt there is any. What I didn't know, is this is the only wagon Hornby, or anyone else made, that can run upside down as those wheels rotate in their cradle!

Oddly, there the R number isn't on the chassis of the model. Does this indicate it is from a set?

This model came my way when I needed something for a magazine article. A little negotiation with the shop owner allowed me to swap the wheels under the wagon for some of the dreadful "Silver Seal" sets inflicted on us by the firm in the period. Now, I wanted 'orrible wheels, so someone else has the slightly better set that should have been under this wagon. Everyone wins!