According to the attached Amnesty International publication, Thailand is not a signatory to the 1951 Convention (AI Index: IOR 51/01/89).
A list of signatories to the 1951 Convention published in Handbook on Procedures and Criteria for Determining Refugee Status does not include Thailand (1988, 87).
Nor does the list of signatories to the 1951 Convention published in The Refugee in International Law include Thailand (1989, 304).
Further information on the subject is currently unavailable to the IRBDC.
Bibliography

Amnesty International. (AI Index: IOR 51/01/89). "International Conventions on Human Rights and Humanitarian Law: The Case for Ratification: Update No. 3."

Goodwin-Gill, Guy S.. 1989. The Refugee in International Law. New York: Oxford University Press.
Office of the United Nations High Commissioner for Refugees. 1988. Handbook on Procedures and Criteria for Determining Refugee Status. Geneva: Office of the United Nations High Commissioner for Refugees.
Attachments
Amnesty International. (AI Index: IOR 51/01/89). "International Conventions on Human Rights and Humanitarian Law: The Case for Ratification: Update No. 3.", 1, 2.
Goodwin-Gill, Guy S.. 1989. The Refugee in International Law. New York: Oxford University Press, 304, 305.
Office of the United Nations High Commissioner for Refugees. 1988. Handbook on Procedures and Criteria for Determining Refugee Status. Geneva: Office of the United Nations High Commissioner for Refugees, 86, 87.