From: Rabbi Mordechai Leib Winkler
To: David Yehuda Leib Silberstein
The responsum discusses the ruling of the Tur Yoreh Deah 184 regarding a woman in seclusion who did not check herself during her period, and the contradiction raised by the Beit Yosef from the Gemara Niddah 9a.
Source: Yoreh Deah I §101