Igor, from the plans I have, the C and D models had the window on the bottom of the fuselage but I can't find any photos. I've got one photo of an aircraft without the Test Center tail marking which is something I definitely don't have and the one I plan to do. Also, according to Squadron "F2A Buffalo in action" it was discovered that the paint scheme wasn't in RAF colors but Olive Drab 41 and Medium Green 42 over Silver Lacquer.
Geo