This product is either discontinued or we cannot get that from suppliers.
Poly Plantronics Spare Battery 64399-03, Proprietary Battery Size, Capacity 190 mAh, Lithium Ion (Li-Ion), Usage Headset.
The Poly/Plantronics Wireless Spare Battery 64399-03 is designed for the wireless headsets current model (2015) CS510, CS520 & the W710, W720 and older models CS351N & CS361N, alike.