The Headpiece, Hospital Collective Protector, M13A1 is part of the Filter Unit, Gas-particulate: Hospital, Six-man, 12 CFM, ABC-M7A1.The M13A1 would be connected to an M2A2 air purifier via a hose attached to the 60mm threaded inlet valve. Six M13A1 headpieces are included in the ABC-M7A1. The M13A1 is stored in an M12 Protector Hospital Headpiece Carrier (pictured).

References 

 (1972). Filter Unit, Gas-Particulate, Hospital, Six-Man, 12 CFM, ABC-M7A1. Washington, DC: Headquarters Department of the Army.