Complex sentence: Koch defended Germanic black-letter script in the journals and publications he made frequent contributions to, and held exhibitions put on by his group Offenbach Schreiber (which promoted hand lettering and calligraphy) in which he expressed the revival of traditional lettering.
Split into simpler sentences: Koch defended Germanic black-letter script in the journals and publications he made frequent contributions to. He also held exhibitions with his group "Offenbach Schreiber," which promoted hand lettering and calligraphy, and in these he expressed the revival of traditional lettering.