External validation comparing the distribution of the AC20 RC stars on a l Kiel diagram. In grey is the full distribution from APOGEE-DR16, and the green contours show common stars between our RC sample and APOGEE-DR16. Red contours show common stars between L20 and the entire anticentre sample used here. The blue contours show common stars between L20-‘Tier I’ (i.e. 20% contamination) and our RC sample. The black contours show common stars between L20-‘Tier II’ (i.e. 33% contamination) and our RC sample.