Thanks for your update , sellsfish .
I'll speculate that what you're seeing , in the looking-glass , is an etching ( one quite possibly sand-blasted ) that has got alot of fine texture in it .
I believe it's mirror-coated , done at the same time as was the rest of the mirror , mostly .
The optical illusion is effective , for the non-polished glass areas appear to be lower-gloss metal (which , of course , it is ) .
I'llk poke around some more .