Some Horstmann back marks...
W. H. HORSTMANN / PHILA. 1820-1850
W. H. HORSTMANN / NEW YORK 1820-1837
W. H. HORSTMANN & SONS 1843-1863
W. H. HORSTMANN & CO. / PHILA. 1864-1866
W. H. HORSTMANN & CO. / NEW YORK 1837-1847
HORSTMANN BROS & CO. / PHILA 1859-1863
HORSTMANN BROS & CO. / NEW YORK 1850-1852
HORSTMANN BROS & ALLIEN 1852-1877
HORSTMANN BROS & DRUCKER 1844-1850
HORSTMANN / PHILADELPHIA 1893-1935
Yours has Philadelphia spelled out...
It's a trolly uniform button... figure out the initials and you'll get the rest of the mystery.