Serial NAND EEPROM (1Gb): Kioxia TC58NVG0S3HTA00 datasheet

Debug headers factory firmware bootlog

Once you plug in your modem to the cable network, your ISP will update the firmware to a version that has features for, and is tested to be compatible with, their network (it's part of the DOCSIS spec, specifically the "SwUpgradeFilename" option in the config file. See here for details).

My modem, once it upgraded to the Xfinity firmware, no longer shares its bootlog to UART on either debug header (as verified with a logic analyzer, both attached and unattached to the cable network).

There is a silkscreen arrow pointing to one pin on each of the debug headers. The pin on the opposite end, farthest away from the arrow, is ground.