Just worked thru this process with a customer whose wait for his RC2 went past 9 months. If you've been waiting for too long (whatever that means), then email that NICS liason (sic) email address with your name, date of birth, SS number, and birth state. According to the response my customer received, you do not need any info from the Form 4 such as the control number or can serial number as this information is not transmitted to the FBI as part of the NICS check.
He got two responses back in less 30 minutes - one that stated his NTN was "delayed" and is now "resolved", and another that said that "all NTNs for your name have previously been resolved and returned to the ATF". So now we really don't know what's going on with his application.
UPDATE: About 12 hours after my customer sent off an email to the NICS "liason", we got an email from the ATF this morning with his approved Form 4. The total wait time was approximately 275 days. God bless whomever in the NFA community figured out this puzzle.