I have several sources of backmarks, both US and British. I have not been able to find that backmark. Generally speaking flat buttons range in date from the late 1700s to the 1840s. It was about that time that new button making technology was developed. Some button manufacturers would put a retailer's name on the backmark. I don't know that yours is a clothing retailer but that may be why the backmark isn't listed. Or it could be a small manufacturer that no one knew about.