This got my interest as I remember when I was a kid, my dads factory had two silver LR fire tenders for emergency use on site before the main brigade got there as they had loads of flammable chemicals around.
Just had a quick check through the paint codes listed for all S3 paint ranges used 71-83 from the James Taylor reference book and not found this listed anywhere (they list all separate Berger, ICI, BL and Rover group codes so a pretty exhaustive list). From another reference, MME appears as a Rover colour from 1981 -1989 as Silver Leaf and used on Metro, Montego, Maestro etc. Taylor does mention that "vehicles supplied for finishing elsewhere (eg. by conversion specialists) left the factory with the body panels finished in primer, which was a matt white undercoat". Any signs that your might have been fitted out by someone else who then chose the colour accordingly and stamped the VIN plate. Might explain the white layer of paint you have found. Just a suggestion.
Cheers.Ian