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Could be, my memory is not that reliable these days
Still, I'm fairly sure it was a modern (at the time) Wessex train. Maybe they declassified one for an experiment? Although I also thought it was an 8 seater as opposed to the 6 seater 1st compartments.
The 6-seater compartments and corridor didn't run the full length of the carriage. Behind the driver's cab there was a small first-class open area with 2+2 seating identical to that in standard class, apart from the cloth colour and anti-mocassars. Perhaps this is what you are thinking of? I don't recall it ever being declassified, however.