RECTANGULAR - Replacement mirror face with or without silicone adhesive.
Note: Silicone tube holds 50g of clear silicon. 
To remove the old scratched or cracked mirror, you need to heat the stainless steel backing to a point where the silicon glue loses it’s strength and a pointed metal probe /screw driver can be used to lift the glass mirror away from the metal housing. 
Once removed, all the old glue/silicon needs to be scraped away and then the metal frame housing cleaned with methylated spirits and then left to dry. 
The new mirror glass needs to be glued in place with some pressure on it, then left for 24 hrs before using.