This is the original seat pan and trim... albeit with a repro seat cover. There was no seat cowl on the 76. I believe first year to have cowl was 78 (400E). This is however, the first bike ever to offer mag wheels as std. equipt. Recently put it on the scale. 356lbs. soaking wet. If I was really chasing performance I think one could easily get it down into low 300s.
Hmm, the cowl must have depended on market destination, I was actually working part time at a Yamaha shop whilst going to Uni when the RD400 was released and I'm pretty sure that they had cowls here form day 1.
The main reason I am so sure was the abrupt styling change that Yamaha underwent in 1976 from previously building bikes that looked like British bikes to building (in my opinion anyway) much better looking bikes that unashamedly took their styling cues form the ducati/Moto Guzzi/Laverda/Morini/Benelli bikes of the time.
Yamaha released a "family" in 1976, the RD 200 and 400 and the XS400/500/750 which shared common styling cues as above.
Courtesy (I think) of Wikipedia, many RD400 owners believe that their model was the first production bike to wear cast alloy wheels, this is incorrect.
The first model XS750 in fact beat the RD400/200 and the XS500 to market in 1976 (albeit that it was promptly withdrawn due to spinning middle big end bearings (never fully solved) and blowing middle drive gear boxes (which was solved).
Don't sweat it though, there is a collector in England who has been looking for a "C" model XS750 for decades and cannot find one. Yamaha themselves do have one, but it was used as a test mule for the D and 2D update testing and the engine specifically is not a "C" model.
Whoops sorry for the digression.
Not to detract in any way form the fact that the RD400s were exciting bikes and very well spec'd for their time.