I’m fairly sure those wheels arrive painted silver and are then painted to match the car, so I don’t think you’ll have any corrosion issues. I’ve had my wheels off several times and although the inside finish isn’t the same standard as the outside face, I don’t recall having blemishes like yours.
In Morgan’s defence, very few manufacturers finish the inside surfaces of their wheels, yours are still better than most mainstream brands would supply.
My advice, forget about it, rather concentrate on keeping the steel hubs and brass nuts in good nick, the steel centre will rust if neglected. If it really bothers you, get some brush on paint to match and fix it yourself.
