ceylon is the former name of sri lanka. In ceylon 1964, there was pact signed between indian former pri minister lal bahadur shstri and celon pm siri mao bhandarayna called as sirimao-mao shashtri pact 1964.
In context of srilanka where we say persons of indian origin, historically most of them are from present day Tamil Nadu.
These persons are desendents of workers who were taken by britishers to celyon in 19th centeury.
these persons were taken to do work in hills, plantation of coffee, tea, rubber in Sri Lanka. when Ceylon became independent as Sinhala control the national government was formed.
As a result many persons of Indian origin were not given citizenship, to resolve the problem which were resulted because of the policy of government of Ceylon an agreement was signed at the level of PM.
objective of this pact was:
All this things are to be carried out in period of 15 years from date of agreement.
remaining people handled with different agreement called sirimao indira gandhi pact 1974. According to pact 75,000 (50%) people will be given citizenship in srilanka and remaing 50% will we given in india.
overall if you see at indo-srilankan agreement of 1964 & 1974. india agree to issue citizenship to 6 lakh persons and srilankan government agreed to issued citizenship to 3,75,000 people.
because of various issue and ethnic violence in Sri Lankan, various reports say that commitment under 1964 agreement were only implemented in part in aggrieved period of 15 years.
some sources say that by year 1985 india granted citizenship to around 6 six lakh persons while sri lankan agreed to accept around 4,50,000 lakh individuals with children.
this matter was recently matter of debet deu to NRC, NPR issue.
it was reported that some families who have became citizen in india after coming from sri lankan persons of indian origins reportedly lost there document in transit.