30HD BSI Stamp

I am confident we can help, but that stamp code only tells us what the strength of the balance's spring might be. That stamp is used normally with an S751-31, but you'll need to carefully compare the entire balance, including its length and the 15-014 shoe, to make sure this matches exactly what you have. If anything seems off, we need to see it out of the window, with pictures of any fittings or attachments on both ends.