I got a question from someone to day about what to do if their Class 3 registration papers are lost. I found the answer on the ATF website under the NFA Handbook in Chapter 3.
Section 3.5 Lost or stolen registration documents. A person possessing a firearm registered as required by the NFA must retain proof of registration, that is, the registration form showing registration of the firearm to the person, which must be made available to ATF upon request.53 If a registrant discovers that a Form 1, 2, 3, 4, 5, 6A, or 10 is stolen, lost or destroyed, the registrant must immediately report the theft, loss, or destruction in writing to the NFA Branch.54 The report must contain the details of the situation. ATF will issue a duplicate copy of the registration document as the circumstances warrant.
If you need to contact a local ATF field office follow this link for the AFT field division contact information.