We cannot trust websites for backmark dating. According the book "American Military Button Makers And Dealers; Their Backmarks & Dates" in the backmark PHOTOS section, that particular version of "HORSTMANN BROS & CO./(middle-dot)PHIL.(middle-dot)" backmark dates from 1870-1893. That date-range matches up with the time of the People's Passenger Rail Way Company.
Ahh, yes, thank you for the correction. I'd mentioned this being found online for just that reason (that it's not trustworthy). I just can't get to my reference books while my daughter is sleeping.