"vacuum desposited mineralization" means an aluminium coating evaporated onto it - since that's the standard way of making a mirror for about the last 80years that's not too surprising.

You can put a coating on the front of a surface - the advantage then is that the base material doesn't have to be transparent. But it is liable to get scratched.

Or you can coat the back of a transparent material. This is how almost all mirrors are made unless you work in an optics lab. The disadvantage is that a small amount of light (6-7%) reflects off the front surface - which doesn't matter for a rescue mirror since it's going in the same direction as the main light.
The advantage is that the mirror surface is sealed and so can't scratch. If the seal is airtight the reflectivity is higher than for a front surface mirror since aluminium drops to about 60-70% reflectivity on contact with air.

The advantage of a glass mirror is that it can be made stiffer and flatter than plastics. Any surface error or overall curvature spreads the beam out and so reduces the power that hits the eyeball of the observer.


ps. Yes that is how you spell aluminium