The seats for the 77-78 models are different from all the rest of the Cb550s due, in part, to the larger tank.
The shape of the back end is how a 78 seat it is identified. (which is not in your pictures)
However, I'd guess from your picture, that either your tank is not mounted forward enough, or you have an earlier 74-76 K seat that only mates properly with the smaller tank on those models.
Cheers,