Judging by the proofmarks, Kevin, you have an "ordinary" WR Explora, rather than the Super Magnum Express, the magnum version. As you may know, Holland & Holland, that acquired Lt. Col. Fosberry's patent for the original version of the rifled choke gun, has resurrected their "Paradox" and is making it and the ammo to go with it. As I recall it fires a 735 grain conical projectile. Suggest you buy some and try them, hoping they regulate. Not sure who sells it, but Lewis Drake has much of the Holland ammunition, as I recall.
Otherwise, you are going to have to try a number of commercial sabot and slug rounds and hope they are a) not too much pressure for your old gun; and b) that one of them regulates. Alternative is to see what works well enough to use the double as a single shot...
Regards, Tim