Help! Two primary Hallmarks; No Eagle or Letter/Number

[twodestiny9s]

I haven't run across a Mexican Sterling "Trembler" Brooch like this one.

This is a front view of the piece:
http://imageevent.com/twodestiny9s/silv ... =0&z=2&l=0" onclick="window.open(this.href);return false;

This is a back view of the piece:
http://imageevent.com/twodestiny9s/silv ... =0&z=2&l=0" onclick="window.open(this.href);return false;

There appear to be two primary hallmarks; neither of them in Hougart's Little Book.
http://imageevent.com/twodestiny9s/silv ... =0&z=2&l=0" onclick="window.open(this.href);return false;

Could it be possible that:
a)the piece is pre-1948 given no federal hallmark?
b)each piece was done by a different artisan and then assembled?

Has anyone seen any jewelry piece like this?

Thanks in advance....
Steven