Tefillin Written on Residual Duchsustus Layer

Description

Rabbi Landau asks whether tefillin are valid when some letters were written on a thin unscraped remnant of the duchsustus layer atop the klaf parchment. The Chasam Sofer rules the tefillin are kosher, arguing that the essential halachic requirement is only to remove the outermost flesh-touching surface, and any remaining thin inner-layer residue does not invalidate the parchment.