There are different kinds of mock up. There are the publicity mockups intended purely for show, then there are engineering mock-ups used during early development process to test for fit of various components before the first prototype is assembled. It appears to me the fasteners and the fit and finish inside of the weapon bay is much too crude to be part of even an hand assembled prototype. It is probably part of engineering mock to test the initial fit of components around the ventral parts of the mid-fuselage, inside the weapon bay, maybe inside the engine bays and main landing gear bay as well.
It seems to me that mid-fuselage engineering mock up may have served its original purpose long ago, so it was repurposed to become part of a publicity mark up. The front fuselage forward of the air intake, the dorsal surface of the mid fuselage, and the vertical tails seems certainly to be functionless publicity mock ups devoid of the surface details of any real prototype. The area around the cockpit should be stuffed full of avionics and thus covered in various access panels. The landing gears are not even a respectable effort to get the the rough appearances right.
Also, I would expect the surface of a real early prototype to be rougher than service aircraft, not smooth like that. Much of the detailed surface treatment for signature reduction for a service aircraft would be unnecessary for a prototype.