This is what 45-pound barbell plates looked like way back in the day. If you have some, count yourself lucky, they started disappearing in the 1960’s when The York Barbell Company came out with a more streamlined plate (they could only fit so many of these on a bar with guys like Wilbur Miller around). Two great grip strength challenges either to lift one of these plates by the hub or pinch grip a pair of them.  You’ve got a pretty strong pair of mitts if you can do either (or both.)