When you put it altogether and shake it up, the ESV felt (in line with its translation procedures) that “letter” was too difficult in v 27 and went with “written code.”
If you have even a teensy bit of understanding of New Testament Greek, Bill Mounce is going to help you with his weekly posts on Koinonia. Here's the latest.
You can work out whether his argument succeeds, but it helps understand why, time and again, the ESV footnotes make the ESV a translation worth preaching on.