The phrase "original signatories" refers to the initial people or parties who signed a document or agreement when it was first created. Full definition
The fork, which was abandoned in November due to a lack of consensus among original signatories of the New York Agreement, proposed an increase in Bitcoin's block size from 1mb to 2mb in an effort to improve its scalability. (ccinews.net)
In Canada, our specific concerns and proposed amendments were largely ignored by the original signatory to the agreement, Stephen Harper's Conservative Party government. (tradejustice.ca)
But two of the original signatories, the GDR and the Soviet Union, don't exist anymore. (politics.stackexchange.com)