This design is from an antique British button from the late 1800's to early 1900's, possibly a livery button from a private estate. 

Monogram: BB

Accents/Accessories: Attached bail; Detailed edging; Raised monogram in center; Flower on bail; Heavily oxidized

Size: 31 x 40 millimeters (1.22 x 1.57 inches)

Weight: 18.70 grams (0.66 ounces)

Sterling Silver Chain as shown: 16 inches - $25.00 or 18 inches - $35.00