Interesting idea, tambo, but have you ever heard of gold being plated with platinum? Why would anyone, let alone Mappins, want to do that ? The idea of plating in jewellery nowadays is to cover a base metal of no worth with a thin layer of a valuable, attractive, metal to give the impression that the whole piece is of the valuable metal. And why use 18 carat gold, at the price it was? Or any gold,if it's never going to be seen?
Mappins always traded primarily as retailers. No doubt they still had in house craftsmen in 1964 for big commissions (enormous silver centre pieces, special trophies and the like), but the standard items, like rings, would be made for them. HLP would, in all probability be the maker's mark of one such contractor. The guarantee that the buyer had, when dealing with them or Aspreys, or Tiffany, or Garrards or other big names, was always that the stones would be top grade; no emeralds which were heat treated or oil filled, and no big diamonds which were only of dress quality, for example. That's why an old piece marked with their names always fetches a premium; nobody even thinks to get the loup out !