I have a C Rapide RFM with straight seat brackets. I think that they look rather like an add-on, and are not nearly as elegant as the curved ones. By the way, what could Irving have been thinking of when he defended that brutal seating arrangement, whereby the poor passenger was guaranteed a luxurious ride by only being subjected to something like HALF of the road shocks. My C Rapide (with curved seat brackets) had the original arrangement for a very short time before I fitted a straight tube from the seat rear to the rear of the engine as per Series D.
-Phil