Great stuff, and good to know you can help each other out.
Don't know if it'll help, but I have seen it in writing, and also been told by a former P51 pilot (RAF, Mustang III and IV), that the yellow line enclosing the 'Basic Six' was in fact yellow tape, possibly insulating tape. I do remember seeing a photo at one time, where it appeared the line was 'sagging', which presumably was the tape lifting, or peeling, at that particular point.