Apple tends to adjust things over the course of production of any particular model. Thus, by the end of production of that model you're getting the benefit of what they've managed to fix or streamline.
Other than the iPhone 5 version for T-Mobile which released about 6-9 months after the launch of the iPhone 5 I can't recall any specific revisions. Apple may classify them as such internally, but they really don't want the customer thinking about that. To the customer it should just be 'iPhone'.
Mostly, variations between carrier are what customers focus on though. Some iPhone 7, 8 and X models had Intel modems and others had Qualcomm.
With the iPhone 6s, Apple had two chip makers: TSMC and Samsung. Everyone wanted TSMC because the perception was that TSMC was the better chip - even though Apple tried to claim there was no difference.
In regards to Qualcomm and Intel modems, there were some rumors that Apple was limiting the performance of the Qualcomm modem to match that of the inferior Intel chip so there wouldn't be an issue with customers (the whole TSMC/Samsung thing being an example).
No one knows for sure on that though.