I have returned 10 iPhones before due to the interlacing scan lines issue. It is annoying and I believe a true defect. Why? Because it's not retina when you see all kinds of lines, especially in pictures. The 6s plus is the only phone that doesn't have them. It does not seem like a coincidence that all of them had a C serial # that I saw.
For the iPhone 5s the problem is global as I have noticed the exact same issue in different countries. I too independently came to the same conclusion that you did. The C serial #s are the best of the screens, for whatever reason. In more than one country I have taken a peak at the serial #s and seen the interlacing on the units with F (worst offenders and I've never seen one unit without it) D's also have it but far less. The C's are more or less OK with me. I'm glad I'm not nuts.
I have only seen iPhone SE with F serial #s here unfortunately. Never seen a C. And we do not have good return policies here, and ordering direct from Apple requires at least a month before you get your money back, and you have to take your phone to a 3rd party shipper, fill out paperwork, etc. It's not worth it.
Why there is no recall is beyond me. I want to buy an iPhone but I can't. Not when phones costing literally 1/4 the price have a 1080p screen that has zero interlacing. It only seems fair that if Apple is going to build a superior phone, the screen should not have interlacing lines.
If you guys get some grass roots thing to get these screens replaced (most genius people think you're nuts or it's just within spec) or better yet fixed the issue so there is no lottery, then let me know. Otherwise I'm done with the iPhone as this issue is big enough for me to jump ship. I can't enjoy my screen unless I'm reading black and white text.