Common notation for tenor voice prior to 1900, especially if older than that.
See, for example, this TTBB arrangement of Cornell's Alma Mater at Wikipedia .
music.stackexchange.com/users/15536/Kevin_Kinsey
I’m not familiar with this particular clef.
However, judging by the context, this is equivalent to a treble clef lowered by an octave for the tenor voice part.
In choral music today, this is usually represented by a treble clef with an 8 below it, sometimes called an octave clef:
music.stackexchange.com/users/6851/Ben Miller - Remember Monica
This is very speculative and I hope someone can give a more reliable answer.
At some stages in the history of printed music, printers may have only had accidentals available on lines; if an accidental was required on a space, for example low F# in treble clef, they would put it on one of the neighbouring lines instead ( reference - see footnote on page 4).
Perhaps the same thing was true of clefs in your example, and the "D clef" is actually a C clef which the printer was unable to print correctly on the space below.
This would make the part treble clef (presumably sounding an octave lower), which seems right.
The general shape is similar to that of some variant C clefs which are relatively rare today ( image ).
music.stackexchange.com/users/50421/David
This is most likely a C-clef, and not a D-clef.
In the History section on the Wikipedia article on clef, there is an indication that an older way of writing the C-clef assumed a "ladder like form".
In the sample you have provided, the clef is indicating the third space on the staff (and not the fourth line) is Middle C.
This is obscured by the stylistic slanting of the clef.
Wikipedia also has another example of this clef in its article on TTBB choral music, which appears to have held onto this form of writing the C-clef longer than other genres.
music.stackexchange.com/users/50430/nicholas
My understanding is that the C clef was a mobile clef prior to being in a fixed location.
It designated where the C (below middle C) was located.
Much the same as the G clef designated where the G above middle C was located.
If you go far enough back to where the staff was split away from 16 lines, there needed to be a clue to what notes would be represented on the particular staff.
Alas, my understanding may be entirely wrong.
music.stackexchange.com/users/62039/Randy
As can be looked-up in the Havard Dictionary of Music , the C clef has some very different appearances (middle three of first picture), and the middle one matches your example.
I would not pay to much attention to the placement, which seems somewhat high.
music.stackexchange.com/users/2600/guidot
This was the original sign of the tenor clef, that defines that the middle C (C4) is on the 4th line.
The tenor part is transcript to G - but the notation is an octave to high.the soprano voice is missing (cut)
, we see at the sheet copy that it is a SATB setting, where the tenor voice originally was set in a C-clef and now it is transcript in a G-clef, that's why the C-clef is written between two brackets.
Irritating is that it's not a G clef with an 8ve below the clef as the tenor must be transposed an octave down.
music.stackexchange.com/users/56219/Albrecht Hügli
I recently came across  this score  by Henry Work called "Grandfather's Clock". And i was a bit confused at first regarding this clef in the chorus. It seems to be a D clef.   I was wondering if this clef was popular at the time, or was used for any particular instrument?
Where was the D clef used?
music.stackexchange.com
<clefs>
music.stackexchange.com/questions/70859
music.stackexchange.com/users/10585/garyF
 In the  History section on the Wikipedia article  on clef, there is an indication that an older way of writing the C-clef assumed a "ladder like form". n choral music today, this is usually represented by a treble clef with an 8 below it, sometimes called an octave clef. Perhaps the "D clef" is actually a C clef which the printer was unable to print correctly on the space below.   The 'D' is actually a 'C' which the printer was unable to print in the corrected space
In today's choral music, this would be an octave clef on the treble staff. The 'D' is actually a 'C' which the printer was unable to print in the corrected space. In an older style of writing, the C-major sign represents a "ladder like form". However, the tenor part is transposed to a key of G, while the rest remains in their original key. This vital capacity in baritone singers, where one of the major alterations in baritones is the lack of the distinctive soprano voice, which gives them a roughness in the lower notes that is generally compensated for in the tenor and baritone (cut)