LCMS declared altar and pulpit fellowship with ALC in 1969 and withdrew from it in 1977.
This was the most ecumenical movement made by the LCMS. What caused breakup
was the approval of ordination of women in the ALC beginning in 1970. So altar and
pulpit fellowship was short-lived with the ALC. By 2001, the President of the LCMS
would declare that the ELCA (a merger of the LCA & ALC) was heterodox.
I saw these posts about the ecumenical contributions of the various Lutheran bodies earlier today but could not post due to a heavy schedule. I'll share a few thoughts about what I saw above. My perspective is shaped by a graduate class on the Ecumenical Movement with Ron Feuerhahn and my own research as a worker at Concordia Historical Institute, as well as I remember.
The debate above about which church body contributed most reflects the perspectives of the different groups involved. Churches that merged to form the LCA and ALC had long experience with mergers, having gathered up many of the little ethnic Lutheran churches to form larger groups.
The LCMS did not have this experience. There are few mergers in LCMS history, just the German and English halves of the synod, plus the Slovak Synod, and maybe one more. The LCMS did participate in the Synodical Conference but that was a conservative fellowship that came apart as LCMS leaders sought fellowship with the ALC.
Feuerhahn, as I recall, called the LCMS leaders frustrated ecumenists. The wanted to merge with the ALC to form a large conservative-right Lutheranism in America. But all fell apart since they lost the Synodical Conference and the ALC. The hymnal was part of this effort for conservative-right Lutheranism, I think.
After the Walkout, the AELC split from LCMS provided an important impetus toward forming the ELCA. So indirectly through the AELC, the LCMS had a significant influence on the development of the ELCA. But overall, I think the congregations that became ELCA had a much longer history of ecumenism than did the LCMS.