⌂ Ladder-style C clef
From Wikipedia:
In printed music from the 16th and 17th centuries, the C clef often assumed a ladder-like form, in which the two horizontal rungs surround the stave line indicated as C: ; this form survived in some printed editions into the 20th century.
From lilypond-user discussion, 2016-09:
Ladder-style C clef for TTBB men's chorus
