Looks like the seat is not original but rather a custom job. Reflex and Silverwing have 2 support sections for the back rest, the yellow Silverwing seat only has one as part of the custom seat. Looks tall enough to support the middle of the back just below the shoulders. Didn't notice until you mentioned it. Looks very comfy. From looking at pictures of seats from two notable custom seat makers Russel and Corbin, this appears to be a Russel Day-Long. Some pictures here:
A bit pricey for my tastes. ($525 to $830 + more $$ for extras) But DANG! they look nice!