My first riot gun was a JC Higgins (made by High Standard) vent rib field gun with a "Dial-A-Duck" that I cut back to the rib post nearest to 18". (I believe the barrel wound up at 19"). Rugged, well made gun. It also did not have a disconnector, and was very fast to cycle and shoot. Just hold the trigger back and pump. Mine was totally reliable. After using it for a few years I traded it off. Wish I hadn't. It was better than the riot gun I replaced it with (Winchester 1200 Defender). In turn I traded it off quickly for a Mossberg 590 and never looked back.
Your rifle sight High Standard uses the Flite-King Brush barrel with an extended mag. Nice find. I like it the best of the three.
What are the markings on your Stevens 620? Some of these were made for military contract and were issued to civilian guards at manufacturing plants.