As I understand it,Jock, the marks on the back are pre 1900, Glasgow Hallmarks STG SIL representing Sterling Silver and T U gives the date.
Given the the cost to have some dies set up and the casting of one of these, you might think it not that common for someone to undertake, as there were makers mass producing clan badges already I'm sure. But you have asked some good questions here. We will have to see what unfolds with more research.

The translation you came up with Shane is one that I got as well. So thank you for that. What strikes me strange is that it is similar, but not a direct translation to Hold Fast, which translated to

Cùm gu sgiobalta.