The 621 is the flagship pencil for the Nataraj brand and one of their oldest pencils. Depending on the market you’re in (or obtaining your pencils from), the imprint and color availability can be different, but the name Nataraj 621 is the one constant. According to the Hindustan website, the 621 is available in multiple colors for the international market, including green, blue, silver, and white, in addition to the standard red that’s available in the Indian market. The red is called “Ruby”, and sometimes I’ve seen this pencil reviewed as the Nataraj Ruby 621.