Here’s what musician Damian Primis has to say:
“One of the challenges of playing the Classical contra is the lack of keys. There are seven tone holes that are covered by the fingers and one by the right hand thumb. The holes are large and if you don’t have big enough fingers you won’t be able to cover them. But at this stage of the development of the instrument keys would not have worked. You need to be able to adjust how much the tone holes are covered to adjust pitch from note to note. Covering the hole all the way, half way, quarter of the way etc.. The instrument is very sensitive with the intonation.
“The modern contra is completely covered with keys. As for the length of the instrument the only difference is on the modern contra it wraps around the instrument instead of going straight up in the air!! So the actual length is about the same just less visible.”