Oddly, on this link it claims that on 2 Sept 1940, Barton's plane, V6625 was coded US-K as the plane had just been taken over from 56 squadron. If Barton was shot down in that plane on 5 Sept, is it possible the plane still had those markings? (Search of V6625 on the link).
http://surfcity.kund.dalnet.se/commonwealth_parnall.htm