Hello
The mirrors back side is hollow beneath the surface — to struck there is need sensibility; because a hard strike could damage.
I guess that the
Z-mark is the partly struck maker’s mark from »
Koch & Bergfeld« - see the following excerpt of mark, from a Hand book for Professionals, from 1903:
»
Koch & Bergfeld« made also in their hollowware range items like your
mirror.
I haven’t a catalogues from this range of products — in the style the mirror looks like to be from the time period between the two World Wars.
»Koch & Bergfeld« is still in business — and in their archive could be found the model number as well (of course, if it is a
K&B product, as I guess).
Look here:
http://www.koch-bergfeld.com/
I think they would help you, to find an answer.
But because mirrors are »
hollowware«, maybe you must take contact to the »
corpus« section?
http://www.koch-bergfeld-corpus.com/
Sorry that actually I couldn’t do more for you.
Kind regards silverport