The Model 520 is built like a Tank. It was manufactured from 1912 to 1932 and still assembled from fabricated parts well into the mid 30's. If yours is marked J. Stevens Arms Company it is post WWI. Stevens made 191,000 of them and the Military bought several thousand.
The Model 520 was originally chambered for 2 3/4" shells so you should have no problems with any standard 2 3/4" loads.
Take down is simple and the gun has no disconector so if you hold back the trigger, it fires every time you rack the slide. Not a bad feature in a home defense shotgun.