Mexico Mine & Smelter Supply Co.

Avenida Juarez, 6 & 8, P.O. Box (Apartado) 447, Mexico City

This firm produced perfins from 1903 to 1910.

Here are two Mexico Mine & Smelter Supply Company covers produced in that time:

Mexico Mine and Smelter Supply Company cover

Receiving cancel (on reverse) 9 April 1904 to Fremont, Ohio, USA
M.M./S.S./C. perfin on 5 centavos Coat of Arms definitive (1903) - Scott 307
Mexico Mine and Smelter Supply Company

Used 18 March 1909 to Worcester, MA, USA
M.M./S.S./C. perfin on 5 centavos Coat of Arms definitive (1903) - Scott 307

Here are some of the stamps which have "M.M./S.S./C." perfins:

Mexico Mine and Smelter Co. 1903 perfin   Mexico Mine and Smelter Co. 1903 perfin   Mexico Mine and Smelter Co. 1903 perfin   Mexico Mine and Smelter Co. 1903 perfin   Mexico Mine and Smelter Co. 1903 perfin   Mexico Mine and Smelter Co. 1903 perfin