Looks like a Savage M720 if you ask me. it could still very well be an a-5 or model 11 for that matter. Definitely not worth that much though.
Plenty of custom work on it, including the checkered dust cover and the compensator. Probably the stock as well. Looks smoothed to match a deeper trim on the bottom of the receiver. Looks like the receiver was re-blued, and when it happened, I think they removed the serial number and ID markings as they slimmed it down. Without seeing it disassembled in person, you likely lose all hope of 100% IDing it. Honestly, with all of that work to remove identification characteristics, I'd wager it was first owned by a shady individual.
My guess is that the curious butt stock and all was fitted to an individual owner. No idea why you're slapping that compensator on a 20 gauge though.