78 ( I don't think it was 77) has a stepped seat, unlike the rest of the 500/550 bikes, it was a one year only design change.
Personally I find them too restrictive, being 6' 4" I need a little more room away from the tank to be comfortable. Stepped seats do not afford that option.
I think Honda largely went away from the bench seat, it was associated with being a UJM or old bike.