Stevens 520 and 520A, and their brand names were made in 12, 16 and 20ga. I've owned many of them in all 3 gauges. Some with the solid rib too. One solid rib gun was 'Riverside' branded.
I've had Sears as well as JC Higgins plus MontgomeryWards, Ranger & Riverside marked guns in addition to the Stevens marked ones.

The stepped topped (and bottom) receiver is the earlier version.
The 520A is the straight top & bottom edge and came out later,,probably mid 30's best I can reason but it's just a guess.
The US Military resurected the 520A when they needed riot and trench guns for WW2 and put Stevens(Savage) back to work making them. I understand the model production had been actually been discontinued. The 620 was already in production.

The earlier versions use flat mainsprings. Later coil torsion springs for mainsprings (520A).
Lots of small changes inside. Not all parts are interchangeable.
If there's anything that breaks often it's the carrier spring (flat spring & two different types) and the firing pin.
The op rod gets worn and disconnects from the bolt at the beginning of the return stroke in a well used gun (hold the gun upside down and operate the action to see if it does). Can be fixed pretty easily if bubba hasn't gotten to it first

There was even a short lived version (520) with a hangfire safety for a while. You need to use the slide lock latch to open the gun even upon dry firing. Marlin hammerless pump shotguns all had one too.

The Remington M17 was also the basis for the Remington 'Whippet' shotgun.
A pistol grip short barreled no-nonsense Security & Law Enforcement defender weapon probably as a rival to the Auto-Burglar and others.
One accessory was offered for it was a belt clip attachment carrier of sorts. The trigger guard screw was replaced with a special slightly longer & extended larger headed screw.That snapped into a spring clip holder that was slipped onto the owners belt. A handy way of carrying the blaster under a long coat common for the day.
A similar 'holster' was offered for Colt SAA's way back too. I can't recall the name of it. The hammer screw on the Colt was replaced with the special lengthen & larger headed screw.

My late production M17 had one of these extended belt holster spring clip screws on it holding the trigger guard assembly in place. Probably just what ever was available for assembly at the time.