Complex sentence: In the exhibition catalogue the "Janssen portrait" was tentatively identified as a portrait of the courtier, poet and essayist Thomas Overbury, but the catalogue asserted that this was simply a guess.
Split into simpler sentences: In the exhibition catalogue the "Janssen portrait" was tentatively identified as a portrait of the courtier, poet and essayist Thomas Overbury. Nevertheless the catalogue asserted that this was simply a guess.