The 'C' for Contax is always near the base of the barrel. To my knowledge, it would only be on 8.5cm, 10.5cm and 13.5 cm lenses. There'd be no reason to put it on a 3.5cm lens. Even 5cm lenses can be interchanged if used at f/2 and above. I shot a sonnar 5cm on a Nikon and a Nikon 5cm on a Kiev for a considerable amount of time before becoming aware of this weirdness. It probably contributed to some missed shots wide open.
On the other hand, I've shot a Nikkor 3.5cm f/1.8 wide open on a Kiev with a Contax mount, and never had any focusing problems. So the f/2.5 lens should do fine.
By the way, it's a dynamite lens. . . .