For mail sent in the early 1960s, postmarks of the Indian post offices handling the international mail to and from Bhutan can be found on covers mailed during that period, while postmarks of Indian post offices assisting Bhutan P&T with domestic mail routed via road or train in India were also attached to covers handled through that system. Leo van der Velden and Aranya Dutta Choudhury, 'Note on the use of Indian post offices in WestBengal and Assam in the 1960s as part of the Bhutan postal services', pp 4-5 in Postal Himal, No. 173, 1st Quarter 2018
