WHAT ARE DEMO/DEMONSTRATIVE LENSES ON EYEWEAR FRAME?
Demonstrative/Demo lenses are put in the factory for demonstration in the finishing process when they intend to sell the frame with prescription purposal, but obviously one can put also common non-prescription sun lenses or clear lenses.
Then they should be changed with proper sun, clear or prescription one.
They are just for "demo purposal" and as dummy lenses made with cheap plastic with carved brand name, many times are scratched (due to storage years/handling).
No one demonstrative lens is usable, always need to be replaced.
Don't make adjustments and set-ups by yourself, let people in the trade like your trusted optician do them.
Don't do "cold insertion" of the lenses in the frame, on acetate and cellulose it could damage the frame.