This monogram is from An Encyclopedia of Monograms, J. O'Kane, 1884. 

Monogram: TSG

Accents/Accessories: Square shape; Detailed border; Metal ink accents; Smooth background; Detachable bail

Size: 60 x 47 millimeters (2.12 x 1.66 inches)

Weight: 40.60 grams (1.43 ounces)