A big thank you to Martin for the truly exhaustive examination of this issue. Do you have a copy of Form 78 for X4168? My Form 78 PDF file for Spitfires R6759 - X4778 had X4167 and X4169 but, annoyingly, no X4168. The Morgan and Shacklady Spitfire book, and
http://www.airhistory.org.uk/spitfire/home.htm have an entry for X4168 of: FF 9-8-40 24MU 10-8-40 610S 15-8-40 222S shot down nr Hurst Green Sussex 7-9-40 SOC 11-9-40.
Peter