I would try removing the mirror and see if there is a screw there that threads into the post. If it is an original, I believe the glass is held in by a ring on the circumference.
I bought a NOS one a couple years ago at a swap meet. The vendor said a quick way to tell original from reproduction is by pushing in on the glass.
If it moves inward, it's original.
If the glass is fixed in place, it's reproduction.
