Thanks, It is a domed button. I found some matches. I just can't find a date to the match. I also can't make out the company on the back of the button. It could be Waterbury?Waterbury made some of the Philly Police buttons. The only ones I could find were 1940's but they were more of a flat button top. Your find looks like more of a domed top piece.
Actually, found the Waterbury that matches yours. They don't mention a date for the design though.
Pattern #09966 – PHILADELPHIA Police – Waterbury Button Company
waterburybutton.com