This Panoramasque Chemical & Airbourne Respirator is one of the best examples we've seen. These come brand new in the original packaging and box. Sourced from the polish military, these are in fact manufactured by Fernez/Honeywell which are known for producing some of the best gas masks in the world. 

They have a broad and flexible body to achieve the optimum seal and has an incredibly wide view lens on it. It also has a five point harness on the rear to secure it in place. These masks take a standard 40mm filter.