Hi Nick, with such a small percentage of area it might not be worth getting it re-coated, but if you decide to do it there are a few places that do it.
Two that I can think of are
Francis Lord Optics and Chi Qin Co. or Advanced Optical Components, but their website has gone so dunno whats going on there.
The coating on your mirror is actually Aluminium, its easier(and cheaper) to do than silvering and lasts much longer, even when not overcoated.
Aluminium oxidises very quickly and forms a very thin protective coating of its own, sort-of like a thin natural Anodising, but it's not as protective as a proper protective coating.
Aluminium reacts violently with anything alkaline and also with some acids(like sulfuric, used to give that 'Matt' finish on anodised parts).
The thin edges of your mirror
might be due to the coating process. But I would be surprised if it is.
Coating is done in a vacuum chamber, some very pure Aluminium wire is heated to melting point where it boils off into a vapour in the vacuum and then settles onto the mirror surface. The thickness of the coating varies depending on where the alum wire(s) are placed in the chamber. Sounds like yours might have been done with a single vapouriser directly above the mirror. Thinner coatings can look grey-ish rather than silver.
