Will this battery cover work with the D-12 magneseum Flashpoint battery grip?
Adorama forwarded your question to me because I bought a BL-5 from them. Unfortunately, I am using it with the Nikon EN-EL18 battery in a Nikon MB-D12 grip, so I cannot answer your question. However, I can tell you that before a reasonably priced charger was available for that battery, I used the DSTE branded battery and cover clone (the one with the charging circuit built into the cover) in Nikon's grip without any problem fit-wise. I switched only because the DTSE battery could not provide state of charge information to the camera. Anyway, I think the DTSE and Flashpoint branded products are the same just differently branded. If that is true, and I don't know for sure, then you should be okay.