Whether or not the words "King" can be printed on the package of salted eggs, this can be put. It is just a propaganda slogan and has nothing to do with it.

There is nothing wrong with marking the grade of salted duck eggs, which does not constitute an offence. For example, the price of ordinary and high-quality salted duck eggs is definitely different. If you don't mark the grade, it looks the same, but the price is different, which will mislead consumers.