M4E1: This typically refers to a specific series or model of upper receiver produced by Aero Precision. Aero Precision is a well-known manufacturer of AR-15 components.
Threaded: The term "threaded" suggests that the barrel of the upper receiver assembly has threaded muzzle, allowing for the attachment of a muzzle device or suppressor.
350 Legend: This is a specific cartridge and caliber designation. The .350 Legend is a relatively new cartridge designed for use in the AR-15 platform. It is known for its straight-walled case, making it suitable for hunting in areas with straight-walled cartridge restrictions.
Assembled Upper Receiver: This indicates that the upper receiver comes pre-assembled with certain components, potentially including the forward assist, ejection port cover, and possibly the handguard or barrel.
