George, we do know that Manton's used W&S at least once (above). Depending on memory of other Manton marked cartridge guns, the one above is fairly typical. Does yours appear to be a higher original quality grade? Is it in better current conditiion? We do know it has BV3 for the Manton brand name. 20 bore guns seem to go for about 1.5X - 2X the price of similar 12 bores. If your 20 bore was similar to the one above, I'd say $2250 - $3000. Post photos and I'll try to give you a more accurate estimate.
Look for a S/N on the barrel loop. I'd really like to know if Manton's were using W&S serials or their own.
