It is kind of strange that when 2 or more different printing methods have been used that the catalogues do not bother to give good illustrations of the visible differences between the methods.
The general look of each method is something that no longer belongs to the basic knowledge of a philatelist

Which is a great pity.
The least we can do is give good pictures and point out the differences that are there apart from the specific characteristics of the printing method.
In the case of the 50c Mercury I would have expected to see in this thread both the offset-litho version and the photogravure one!