I'd say trust the position of the flat and call it a G-clef. You can tell this is correct because the C instruments in the score are also in F.
A quick perusal through the score also shows that...
The tenors sing the same lines as the sopranos an octave lower on pages 125-126.
The tenors sing pitches that make sense with the orchestration (see tenors and bassoon 1 on the Db on page 127)
...but this is only true if you interpret this staff as a G clef.
I can't see how any argument about any other clef being valid could be made given this. I was surprised to see that it happens on multiple pages, though, and would be very curious to know how the score was engraved and using which technology. I'm not familiar enough with Norma to know the year, not whether this is the original published edition.