Hilchot Shabbat #104 - SA Siman 293 Saif 1-2 (with Mishna Berura & Sephardi Poskim) [Earliest Time for Arvit Motzai Shabbat, Tzet Shababt] - Jun 13, 2023 - Halachot of Shabbat Shiur #104 - Shulchan Aruch Orach Chaim Siman 293 רצג Saif 1-2 with Mishna Berurah and Sephardi Poskim
Rav Moshe Feinstein regarding rabbenu tam opinion
Maharam Alashker (Siman 96)
Opinion of Rabbenu Tam and the seeming contradiction in Maran here