Here's a snippet from a 1624 facsimile of Juan Arañés' "Libro Segundo de Tonos y Villancicos".
This is a rather syncopated, lively song, with a time signature of C3.
Note values seem to be indicated only by the stems, and my guess is that the filled noteheads indicate duple meter, while the hollow noteheads indicate triple meter.
This works well until the next-to-last measure, which is clearly triple but uses filled noteheads.
Question: Is my guess about filled vs hollow noteheads correct? If so, is the next-to-last measure just an error? (Note also the missing dot in the first measure on the second line).