A 44-year-old non-smoker has generalised periodontitis. The worst interdental attachment loss is 4 mm, radiographic bone loss is confined to the coronal third, and most probing depths are 4-5 mm. However, 36 has a 6 mm pocket on its mesial surface, with a 4 mm deep vertical (intrabony) defect on the radiograph. How should the stage be assigned under the 2017 classification?