Mints are responsible for quality of their product – i.e. – coins they produce, and the mint-mark is a way of tracing the product back to the manufacturer. This is similar to how the packaging of most products mentions the details of manufacturing facility. The mint-mark is not the primary feature of a coin’s design, so it is usually much smaller than other design features of the coin.
The first mint producing uniform coins in India was set up in Calcutta by the British, followed by Bombay and Madras mints. The Madras mint was shut down in 1869 and Calcutta and Bombay mint have been responsible for most of the British India coins production from 1835-1947. During the 1940s, the need for additional capacity led to setting up of the Lahore mint, with first coins produced in 1943. As the oldest mint, Calcutta mint has no mint mark. Bombay Mint has used various marks – during the end of the British era, a dot (.) underneath the date or at some other location has represented Bombay. Lahore mint has marked its coins with a ‘L’ (refer to picture above).Coins of Lahore mint are rarer compared to coins from other mints, and sell for a premium.
In older coins, Bombay Mint has used more exotic markings. During the reign of Edward VII (1901-1910), the Bombay Mint for some years engraved a tiny ‘B’ in the crown on the reverse of the coin, and used a small dot (.) engraved in the floral pattern. The Calcutta mint carried no such distinguishing marks. The ‘B’ is so small that it can’t be seen without a powerful magnifying lens, and so delicate that it gets eroded very fast. (refer to picture below). The coin to left is from Calcutta mint, while the coin on right is from the Bombay mint.