I'm not seeing the face real well, but I will tell you what I think is somewhat correct. From what I can tell it is a Navy button. The HV Allien, London backmark was apparently used after the civil war. Somewhere between 1877 and 1948. Maybe CBG will come along. He has some much better backmark material than I have. I think your button would probably be more towards the early years since the later backmarks were London and NY and then NY.