The picture below shows a larger view of this 1905 Boston Massachusetts Quarter Centennial Advertising Souvenir Metal Pin. This pin has what appears to be the Pony Express horse and rider in the center. The pin may be a postal employee pin. It may be made of silver but it has not been tested. It is in the shape of a flower. And it is marked on the two sides as follows:
QUARTER CENTENIAL BOSTON 1905 P A OF A ROBERT STALL N.Y.
The pin measures 3/4'' wide. It appears to be in excellent or better condition as pictured.