Was the RB Graphic SeriesB ever produced as a "de luxe", mahogany & brass model?Such a camera was recently sold on Ebay [item no.130046514468]for£350-$500+].Could it be a stripped down standard model with all the metal work burnished?

The particular camera that you are referring to is a stripped down model and the hardware has been polished!
On special order, the factory would do anything practical, so it would not have been impossible for such a modification to have come from the factory, but this one is not likely! All of the wooden body models were made with Honduras Mahogany and the brass hardware was either finished with an oxidized silver plating (gun metal type)or a painted finish!
Have a nice day......Bert

Bert is right, Graflex would build custom cameras and even built a 3x4 RB Auto out of Teak for the President of the Pennsylvania RR.
The only way to tell or even suggest that a particulary camera was a factory custom vs a garage custom is to look for material or workings that could not be done by a typical garage woodworker or even machinist. In many cases the factory had machines set up to make easy work of a complicated joint, so what would have been the simplest way for them would be horrifically complicated for a carpenter, or joiner.

This camera has all stock parts that seem to have gotten to close to a wire wheel. The shutter speed plate is now very difficult to read, which again suggests a garage job over a factory job.

I did see what I believe was a factory conversion similar to this. It was a Home Portrait Graflex that had been stripped and lacquered, but the bellows and chimney were of a brown leather identical to brown bellows material used on portrait cameras from the same time.

So the quick answer is, yes, they have been known to make cameras like this, just not this one, and they were never in a catalog.