How is an entirely different turret (and even a bigger gun) less obvious than the gun being the only difference between a regular P4 and a CP4? The T34-85 even has different sounds.
Other mediums/vehicles are a fair comparison. It's just down to personal knowledge to distinguish units based on vehicle model details.
Because the difference between the snub p4 and the long barrel is like 3/4 of the entire barrel. As opposed to the 34 variants which is much smaller gap.