Got it (I think)
“[the motto] properly belongs to the family of Hastings-Forbes, Earls of Granard. The Earldom was created in 1675, so presumably the motto was adopted about this time. Debrett translates it as “The incitement to glory is the firebrand of the mind”.
Compare the coat of arms in this listing:
https://en.wikipedia.org/wiki/Bernard_Forbes,_8th_Earl_of_Granard
Wow! That looks really close! Those don’t look like dragon heads on the shield. Thanks, Etruscan. 🙂 Now I’m wondering if these buttons are mass produced or just made for that family. I guess there’s no way to know for sure.