Question about USAAF ETO lettering

[migmadmarine]

For the individual aircraft letter what does it mean if a letter has a bar over or under it as I have seen in a few cases, such as the 485th P-38 skin that was included with the "lightening" campaign pack lettered "7F-W bar"? does this denote an officer or someone significant, or is it merely a way to differentiate between two aircraft labeled with a W?

[Rjel]

Quite often symbols like that were used to denote a second aircraft in that squadron used the same ID letter. It was common in the 8th AF late in the war when squadron strength was enlarged to more than 26 aircraft. The 4th was large enough at times to fly double missions to different targets. 

[Bremspropeller]

Lt. Sobel says: Letters are contraband!

[DD_Fenrir]

It simply differentiates between two aircraft in the same squadron with the same letter.

 

It often ties in with the initial of the pilots surname; P for Preddy, Y for Yeager, for example. However, this privilege seems to be reserved for element/flight/squadron leads.