If you didn't have a FCC amateur radio or GMRS license and you registered for a PVVET exam, during the registration process, you've obtained a FCC registration number (FRN). If you take an examination with a different VE team and/or VEC, you should use your FRN in-lieu of your Social Security Number on the Form 605.
The FRN is assigned by the FCC and should be used on all applications (for indivduals) that end up processed by the FCC. ALL VECS are required to accept the FCC Registration Number even though some teams also accept SSN's.