I don't know what material was used to make that grey stopper, but I've never seen one that wasn't as hard as a rock, and they couldn't have been that way when they were new. A while back I bought a new old stock one from a long time Maytag shop here in Burbank. It was still in it's original packaging and had never seen the light of day, and yet the stopper was completely hardened. So I'm kind of under the impression that today most folks have these as display items. If you wanted to use it, new rubber stoppers designed for piggy banks fit the hole very nicely, but you'll break the original stopper trying to get it out.