I was thinking about your parts car over the weekend. There was a batch of very special Parts Bin cars produced for Europe (where Rover were starting to be in trouble against BMW) close to the end of P6 production. These had every conceivable extra fitted, including many NADA items which were never listed for the UK. An example would be front door elasticated map pockets. Many of these cars failed to sell and were brought back to the factory and convertyed to RHD and sold into the UK. P6's are extremely easy to convert between LHD and RHD, all you need are a couple of steering boxes, an instrument cluster and a pair of shin bins. Everything else is interchangeable. The give away that this is what has happened will be the position of the fuse box. LHD cars as built have it in the RH shin bin area, RHD cars in the LH shin bin area. Conversions generally didn't bother moving it, so you might well find it on the drivers side on yours.
The one thing that really throws me is that NADA supplementary vent tank. I wasn't aware that any of these were ever fitted after the end of NADA production in 1970! You might also find that it has an electric fuel pump and a reserve/main tank changeover fitted underneath on the off side near the rear axle instead of the usual position (and mechanical pump) close to the front bulkhead. That would confirm a deep raid of the NADA parts bin. I guess it must be possible that some European countries were getting ahead of the UK in emissions legislation in '76. Switzerland springs to mind. But even so the appearance of this particular bit of kit so long after the end of NADA production is astonishing.
There are three people on here who should be able to shed some more light on this - Rudiger, the club archivist, Ian Wilson of Rover-Classics fame who is a bit of a NADA expert, and Nick Dunning who has spent many happy hours with his head in the production records. I've PM'd them to get them to have a look!
Regards
Chris