I’ve spent some time extracting shellball ROM’s for people who have flashed a full ROM, and want to go (semi) back to stock, but have mislaid their backup. For ChromeOS to update, you will need change the HWID to something valid e.g.
gbb_utility –set –hwid="PEPPY A2A-A2E-A5W" bios.bin bios.bin.new
You’ll probably also want to download every file in the relevant directory to ensure the GBB script works. You can use lftp’s “mirror” feature to download all those files quickly and easily. Find the shellball ROM’s @ https://johnlewis.ie/Chromebook-ROMs/shellballs