The "charm" you found is actually a late-1800s to early-1900s Shoebutton-Cover. The attachment on the back of yours is the earlier version of the 1922-Patent version in the image below.
Your button is the latter-1800s version of US Postal Service button. It is shown as button FD-61A in the "Federal [Government] Departments" section of the button book by Alphaeus H. Albert. The "POD" stands for Post Office Department. It was renamed Postal Service in the 20th Century.
According to its EXACT backmark, your button was manufactured sometime between 1870 and 1920. Because there are so many slight variations in Scovill backmarks, the McGuinn-&-Bazelon book on button backmark dating includes photos of various Scovill backmarks, telling the specific time-period of each version. Note that your button's Scovill backmark has a small (lower-case) letter o in Co. That means it was manufactured sometime between 1870 and 1920. The name of the book is "American Military Button Makers And Dealers; Their Backmarks & Dates."