If that is the case, then you would have to play the lottery to get a good one that starts with F1 or F2. My iPhone 6 Plus starts with F2 and I have checked thoroughly and no issues, screen is perfect, no yellow tints, no dead pixels.
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I don't think it really matters where it came from because they will be the same. It depends on what screen manufacture they used.
If this matters:
Nice Name: iPhone 6 plus
Family name:
Group1: iPhone
Group2: 6 plus
Generation:
CPU speed: 1.4GHz
Screen size: 5.5 inch
Screen resolution: 1920x1080 pixels
Colour: Gold
Production year: 2014
Production week: 39 (October)
Model introduced: 2014
Capacity: 128GB
Memory - flavour: xx
Memory - maximum total: 1GB
Factory: F2 (China, Zhengzhou - Foxconn)
Mine starts with F2 and have no issues at all. No yellow tints either.