Prometheus is one of the most famous Japanese brands for aftermarket airsoft upgrade parts. Prometheus have established a name for themselves by providing nothing but the best quality upgrades for a wide variety of airsoft gun types, and this piston is a fine example of that. The polymer body is very strong, and when combined with the 7 metal teeth it makes the perfect composition for an airsoft piston. The reason the back teeth have been kept polymer is so they can flex when, slightly, when the inertia of the gears connect with the piston teeth. The teeth near the fton of the piston are reinforced with metal so they don't give at all as the gear teeth are letting go while the spring pushes the piston forward. Prometheus is the number one name for upgrades to many highly skilled airsmiths, and it's easy to see why.
The use of aftermarket airsoft parts is not an exact science. Some parts may require fitting even thought they were designed to fit without modification. Professional installation of upgrade parts is highly recommended.