That phenomenon is the source of one of my all-time favorite man page entries: https://web.archive.org/web/20210421224904/https://nixdoc.ne...

     ep0: 3c509 in test mode. Erase pencil mark!  This means that someone has
     scribbled with pencil in the test area on the card.  Erase the pencil
     mark and reboot.  (This is not a joke).

It's the E1 pad between the barcode and the empty BIOS socket: https://en.wikipedia.org/wiki/3Com_3c509#/media/File:3Com_3C...