I think not giving the -1c a different icon than the a or d is an admission that the performance of the c is no better, indeed, worse than both of the others. The c is also the plane that has the lowest cost and would most likely not be targeted much anyway.
The other planes have clear performance advantages and should be called out if they are to be in the same arena as other planes.
I believe that all the spits perform enough different to get seperate icons. I believe that all the 109's need seperate icons.
lazs