This helicopter is listed on Rotorspot as a "369ER (500ER)". I have never read about any version of he 500 being designated "ER". That's not to say this version doesn't exist. Maybe the "ER" designation refers to an "Extended Range" version fitted with an auxiliary fuel tank.
JetPhotos.Net Photo » 4X-BDJ (CN: 0315E) Private Hughes 369E by Moti Shvimer
500 Fan.