This is a replacement Bolt Mass Key for the EP45 and EP9 Carbine bolts. Those bolts require this key for the added bolt mass and to retain the charging handle. 

Note: Early EP45 units had a different Bolt Mass Key design which used two springs pins to align the key with the bolt. To ensure compatibility, confirm that your bolt has a channel in the top and not two holes before purchasing this part. All new EP45 bolts sold on our website are compatible with this Bolt Mass Key.