Yes you are correct. It should also have a number on it somewhere, like 10, or 21. If it has adjuster knobs on it, it is probably a Mag 21.
The difference between threaded and threadless is simple. One has threads to screw the top cap on it, and the threadless uses steer tube compression , to sinch the headset down. Threadless headsets are lighter in weight, and come loose a whole lot less than threaded systems. Especially in an off road app.Part availabilty for the Mag series forks is becoming scarce. If you want to keep it, get a rebuild kit, and a pump. Third Hand Loose Screws has some. If not then either sell it or keep it as a spare, and get a Marzocchi!