In the Hasiyum Jr. given out at the Twelfth Siyum says that
Reb Yechezkel Abramsky, Reb Chaim Leib Shmulevitz, and the Gerrer Rebbe spoke at the 6th siyum. It also said the main Siyum in 1968 was in Yerushalayim and at that time the Daf yomi Commission of the Agudas Yisroel of America was trying to spread this idea in America.