I didn't find much on the 511 in the digging I did. My "Gun Digest Book of FIREARMS ASSEMBLY/DISASSEMBLY Part V: Shotguns Revised Edition" states: "Some models of the Fox double, and the Springfield Model 511, have internal mechanisms that are practically the same." My 10th edition "GUN TRADER'S GUIDE" (1983) doesn't even list the 511.
My very humble opinion is that the 311 and 511 are very basic guns that wear their quality on their sleeve. You will never make a great gun out of one but they are reliable and can be shaped and prettied up significantly. Parts are very available and unless cocked for long periods they are reliable. If left cocked for long periods they will require new hammer springs. My 1930's M 311 16 ga is on it's third set of springs (& 2nd butt stock fitted w/a pad). My opinion, if you want a solid inexpensive SxS that should hold it's value but will never be a "good" investment, go for it. About a month ago I grabbed an add off the club wall for a 3", 20 ga, M 311 for $250. I called, but it was too late - Sold! On a private sale, do take 2 to 6 primed hulls to pop off and prove the firing pin springs, safeties and all around functioning. Some of these guns have seen a bunch of abuse/neglect over the last 1/2 century or more.
Good luck,
Scott