'C' Hallmark - can you take a look?

[brocanteur]

Hi all,
I'm looking at this Mappin & Webb ruby brooch, and am wondering if anyone might recognize the thick 'C' mark next to the '750' stamp?
M&W stock very high end brands, and I'm wracking my brain trying to figure out what it might be.
Any help would be greatly appreciated! Thank you.

 

[brocanteur]

In fact someone suggested it may be a horseshoe. Anyone have any pointers?
The brooch is ca. 1990.

 

[brocanteur]

A closer photo... looks like writing in the circle. Does anyone know what it says?