Date: 26 Kislev 5590
From: Rabbi Moshe Sofer
To: Rabbi Sheftel
The responsum discusses the halachic validity of a 'tzurat ha-petach' (door frame) for an alleyway, specifically when the poles are set back from the courtyard wall and a string is stretched across, examining whether it needs to be visible to be valid.
Source: Shu"t Chasam Sofer Collected §34