
This letter is well-known to the collectors of Indian postal history but it is not a good selection to focus on stampless cover postmarks.

The rectangular
INDIA PAID [(Giles SD20a (ex-CX13)/Lowe HS321)] dispatch mark (3cm×1.8cm) of Kolkata, in red, was not exclusively used in the Br. India pre-stamp i.e. pre-1854 era.
This mark was introduced in 1845 but it has been recorded used as late as 1864.
Para. 5 of the Postal Notification dt. April 20,1845 issued by LJH Gray, Postmaster General, Bangla Circle (1844–45) states that
Steam postage on letters for US or British Colonies in America, or the West Indies (unless addressed to Agents in GB) cannot be prepaid in India, whether sent by way of Marseille, or via Southampton.
Also Para 7 of the same Notification states that
Letters for any foreign country, directed to an Agent in England, will ve received or disposed of, as if addressed to GB.
Since this letter was endorsed to get it forwarded by the Forwarding Agent Baring Brothers and Company, London, England to USA from Southampton, via Liverpool, steam postage upto Southampton could be and was paid in India, which was 8A for letters weighing upto 14.18gm (½oz), the rate in effect from March 1,1844 to September 30,1854.