I read this as:Voss and Co. Stolp I(N) P(ommern)

the 1913 Teschner & Collath calalog list only a gunmaker C.Mainzer, wonder if such a little town have had 2 gunmaker. Maybe Voss was the successor of Mainzer?
Pommern is the area on the cost of the baltic see from big Ruegen island going east.