This correspondent is identified in the published Igros Kodesh only as "Michael Yehuda Aryeh Leib HaKohen" (about 5718–5721, 1958–1961). The letters appear to be to one person, active in education and spreading Chassidus in Eretz Yisrael.
Spreading Chassidus. The Rebbe urged him to use Yud-Tes Kislev to strengthen outward efforts in spreading Chassidus (IK 4945), and taught that following in the footsteps of our leaders by spreading Chassidus brings their blessing (IK 7478). Even at times dedicated to remembering the destruction, we must channel them into positive action (IK 7385).
Education and the holidays. He encouraged proactive involvement in education and communal preparation for the Days of Awe (IK 5756).
Shemitah. He warned strongly against plowing during Shemitah, even when others do so (IK 6579).
Sources: the Rebbe's letters cited above.