This monogram is from a 14th century Manuscript Alphabet book which resides in The British Museum in London, England. 

Monogram:  P

Accents/Accessories: Smooth background; Small loose bail; Central monogram

Size: 43.18 x 27.94 millimeters (1.70 x 1.10 inches)

Weight: 7.40 grams (0.26 ounces)

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