37 Fan......welcome to TFF.
OK......here's what the Blue Book has to say about the Remington Model 31:
"MODEL 31A SLIDE ACTION - 12, 16, or 20 ga., side ejection, 2 or 4 shot mag., 26-32 in. barrels, various chokes, takedown, pistol grip stock. Approx. 160,000 mfg., 1931 - 49."
The Model 31 was made with a plain barrel, solid rib, and ventilated rib.
Originally Posted by 37 Fan
The barrel appears to have a raised rib machined into it, not on top like ones I am familiar with.
If the rib isn't on top, where the heck is it!