GRI: Georgius Rex et Imperator: George, King and Emperor: commonly used to mark rifles for Imperial service built at Ishapore Arsenal in India.
I.S.A.: another Ishapore marking, possible Ishapore State Arsenal and likely applied post-1948 Independence.
PRP I have seen before but don't know what it means. Regimental marking, most likely.
It's a genuine Number 1 Mark 3*, made a year after the British quit building them. Many thouands are STILL in Service in India and Pakistan both. The standard of finish was not as high as with the British and Australian rifles, but the materials were the same and they all had to pass the same Proof tests. Stocks look a bit different because of the different type of wood used: you don't find much European or American Black Walnut in India, so they used local woods and treated them to serve as useful stocks.
It can certainly be made to shoot well and, if there is such a thing as a "real" milsurp, this rifle is certainly one of them. Nice old piece; I have her twin. Treat her well and she'll outlast you AND your grandson and put meat in the freezer every year, just as sort of bonus.
.