Gorgeous gas tank, It looks like new. it looks like it might have been a cosmetic dealer take off as it has a extremely low indentation(paint is intact) top front right which is very hard to notice. even looking at it from the top, and not noticeable when looking from the side.
As to the exact model, please compare the pictures to the bike you are trying to match.
The Color Combinationsaccording to the Honda Identification guide is either the 1981 CM400A which was Candy Bourgogne Red, Red Brown Metallic
or the 1981 CM400C Custom. which was Candy Muse red/Metallic Brown