It is the same as the "standard" notation for tenor voices, written using the G clef and sounding an octave lower than written.
The vestige of a C clef on the 4th line (i.e. a "tenor clef") is an indication that this isn't a standard treble clef. The more common notation is a small 8 below the clef.
Looking at the music in the score makes it clear that is the only reasonable interpretation.
If the comment about a "follow up accidental" actually meant a "key signature" since the title of the piece includes "G major", the published score does not use key signatures even though the music key center is G for most of the time.
The SMuFL music font specification names it as "G clef ottava bassa with C clef" - see Unicode character U+E056 here.
Let me start by suggesting that imslp.org is the Number One place to go for clarifications of score markings.
Here is a clippy from a different publisher which makes the notation clear. In this image the more common "little '8' below Treble Clef" notation is used.