A few comments:
If it is 1997 built, then the blank neck plate could be stock. Mid-1997 was when
G&L switched from the 3-bolt to 4-bolt neck and they initially used blank neck plates.
See
When and why did G&L switch to a four-bolt neck?, which is in the G&L Knowledgebase's
General G&L Questions sub-forum.
You cannot make the generalization that if there is no date stamp in the neck pocket, then it is non-USA built.
It is quite possible that the neck pocket date was removed if the pocket needed sanding as part of the neck installation to insure a proper fit.
I agree with Ches that it is not likely that a USA neck of that vintage would fit properly on a Tribute body. However, the
guitar has been modded, so it is quite possible that the neck pocket could have been re-worked to fit the neck.
Once you get it sorted out, please do add this instrument to our
G&L Registry.
Hope that the outcome is what you are expecting and paid for.
Hope this helps.