12.04 - How do I resolve apt GPG error NO_PUBKEY?

I do not recall why I have download.opensuse.org, but I am OK just removing whatever is causing this error:

W: GPG error: http://download.opensuse.org  Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6FE3AE1CC7D68E37

This does not appear to be a duplicate question because the solutions proposed in other similar questions are not working here:

# sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6FE3AE1CC7D68E37
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /tmp/tmp.OCmHCQ908W --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver keyserver.ubuntu.com --recv-keys 6FE3AE1CC7D68E37
gpg: requesting key C7D68E37 from hkp server keyserver.ubuntu.com
gpgkeys: key 6FE3AE1CC7D68E37 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0

I'm on 12.04