This document, in the section running over page 11 onto page 12, refers to Rav Feinstein's chiddush that it is possible to 'convert' to Judaism by chazakah alone, and without a formal beit din. The article adds that similar rulings were made by Zachar Yosef, Maharam Shick and Tzafnat Panei'ach.
I'm aware of Tzafnat Panei'ach saying this (here) but can anyone help with identifying the other two sources?
Many thanks!
