Different locations of C in tire sizes
To answer the auxiliary part of your question first, the 35/43 does not indicate a range of rim widths, but rather an approximate range of compatible tire widths. This is expressed in mm which is why it's the same for ISO and the French sizes. 

The "C" is indeed historical. Back in the day, a 700c tire and rim combination was such that the overall circumference was 700mm. There were also 700a, 700b, and 700d sizes. "D" was the widest tire size and in order to achieve the overall 700mm diameter, a smaller rim was used. Vice versa for 700a's. 

Nowadays, we generally refer to a 622mm rim as a 700c and mount basically whatever tire size on it we want (within certain limitations) and don't worry about the overall diameter. 

I have personally never noticed a "C" or any other letter in an ISO size, but then again I pretty much ignore that. Despite the fact that the ISO is the International Organization for Standards, the de facto standard in bike sizing is the old and admittedly misleading French sizing.