The use of a royal cypher in the Commonwealth realms originates in the United Kingdom, where the public use of the royal initials dates at least from the early Tudor period, and was simply the initial of the sovereign with, after Henry VIII’s reign, the addition of the letter R for Rex or Regina. The letter I for Imperatrix was added to Queen Victoria’s monogram after she became Empress of India in 1877.

Source: Royal cypher – Wikipedia