Looks like the real deal to me Peter, I am pretty sure should an owner wish to have an age related plate, and if the engine and chassis numbers are available, then the MG car club are bound to have access to MG records and could determine it`s identity as original or otherwise and if original then perhaps actual date it left the factory then I suspect there would be no great problem in applying for a plate. Of course the owner could just buy a pre suffix/prefix plate to remove the "P."

Of course if the P is denoting the year it was registered in the UK, then I suspect it is possible the law is being broken in showing non-reflective plates which are required for all cars registered after "L" 1972...?

But then if a Rhodesian issue plate and displaying the NR octagon may be there as a get out of jail free indicator should he be questioned on the plate..?