You have it right. EI means "exposure index." Technically, any time you expose at other than the speed specified as ISO, you are not exposing at ISO. There is only one ISO because that's a standardized speed with strictly defined criteria. Anything else is an exposure index.
It refers to the film speed. You "get" EI 50 with any camera the same way - you take a meter reading, either with an internal meter or, in the case of a Speed Graphic with an external one, and set the film speed to that speed, or adjust from a different setting to get that effective setting. With a manual camera you transfer the shutter speed and aperture for that exposure to the camera lens and shutter.