I don't recognize the 'mark'. It certainly looks like an engravers logo or signiture mark.

I'd contact Roger Bleile at the Firearms Engravers Guild (FEGA)

He's their historian and they have a very in depth collection of material on engravers both famous and obscure including 'signitures'.
He may have something on it.

Include ALL the pics you have to help ID the scroll style and the firearm as well (period of mfg, ect sometimes helps)

FEGA-Historian:
C. Roger Bleile
Historical database.
[email protected]