The Kalamazoo brand, whose guitars had bolt-on necks, filled that slot."USA" was added to the name on the headstock to set it apart from cheaper, imported guitars.
Around 1969, solid state versions of the Models One and Two were issued in very limited numbers.
While the bolt-neck design was already a money saver, Gibson sought cheaper materials as well and found them in MDF (also known as Masonite).
Money was also saved on the pickguard (a single sheet of plastic, not laminated) and the (open-back) tuners.
These were renamed Model 3 and Model 4 respectively.
These models, while novel in their day, ultimately proved unpopular.