This monogram is from the book Monogram und Dekorationen,by W. Diebner (1908). 

Monogram: PME

Accents/Accessories: Oval shape; Raised edge with oxidization; Small jump ring; Loose bail; Mirrored finish

Size: 26 x 16 millimeters (1.02 x 0.63 inches)

Weight: 2.60 grams (0.09 ounces)

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