Just got off the phone with Atlanta. They said I would receive an additional form to fill out proving I'm a US citizen. It seems that on March 26th the form was added to the requirements, but field offices do not have the forms and may be unaware of the requirement. I requested forms from Birmingham in early April they never mentioned it, said send the 7cr and 30 bucks. However the NLC has "a few" copies which they photo copy and send when they reject your application for not having it. It should also be noted, I called on Monday to check my status. Mailed it 5 weeks ago. They said they received it at NLC on 11 May. Realized I needed additional form the same day I called. They also said the processing clock starts over when they get the additional (new) form back at NLC.
UPDATE: Got the letter today the form is an ATF F 5330-20 (3-2002)
Title: CERTIFICATION OF COMPLIANCE WITH 18 U.S.C. 922(g)(5)(B)
It appears I write my name, fill in United States as my country, and sign it.
The letter also said if I fail to return it within 30 days my application will be considered abandoned.