I'm working with a score that at various times has parts marked "Cello", "Violoncello Basso", and "Violoncello Contrabasso" (though not all are present at the same time), all notated in the bass clef. Are the "Cello" and "Violoncello Basso" likely to be the same instrument, with "Violoncello Contrabasso" the double bass, is there a member of the string section I was unaware of (and not mentioned in Blatter, as far as I can see), or is there some other likely meaning?
At least for the distinction between cello and violoncello basso there is one possible explanation (assuming roughly baroque era): Violoncello basso may be part of the continuo group (may also be notated as figured bass) as opposed to a partially solistic / melodic part. (So it are the same instruments, but different players.)
Contrabass is notated also in bass clef, but sounding one octave lower, so I see no reason to assume a different instrument.