Complex sentence: Posing as a porter, Gore brings the painting back to Darcy together with a forged letter and lab report from Carpenter claiming that there is indeed a painting beneath "The Game Bag" which is probably from a fifteenth-or sixteenth-century Italian painter.
Split into simpler sentences: Posing as a porter, Gore brings the painting back to Darcy together with a forged letter and lab report from Carpenter claiming that there was indeed a painting beneath The Game. The original painting was probably from a fifteenth-or sixteenth-century Italian painter.