That looks absolutely fantastic!
There were a few GNR coaches, built about 1954, which looked not unlike that and which survived into UTA and CIE days. Obviously they were only three years old when the GNR was dissolved, and thus could have been expected to last until the mid or late 70s. They didn't - the timber used was very poor and no match for the rock solid GSWR wooden stock forty years older. But a few did last into black'n'tan days (and a few in UTA livery too).
It also looks like one Bredin first class vehicle (I think it was no. 1900?).
That's an exceptionally neat paint job.