My hands are large, but not huge. Certainly not big enough to do any of that stuff your grandfather did. A hex dumbbell is a dumbbell that has a hexagon on each side of it. I've never seen one over 100lbs, because once you get up to that weight they are enormous and impractical to use. They are very common in the States, and are usually the cheapest dumbbells you can buy (cheap in both quality and price). You might not have them in Europe, because I've never seen a pair measured in kilos.
To hold onto these dumbbells you have to spread your fingers out as far as they can go (kind of like palming a basketball). An incredible amount of tension is put on each finger. When you pinch plates or use grippers you don't have as much stress put on each finger because they are all mashed together so the strongest fingers take up some of the slack for the smaller fingers. When you hold hex dumbbells, your fingers are all spread out, so when your weakest finger fails, the whole hand will fail. During very heavy deadlifts, your fingers are all together, but as you lift the weight of the bar, your fingers begin to slip away from each other slightly. When this happens, the weaker fingers will start to slip off the bar (especially the pinky). So, the stronger your fingers the better your grip. That's been my experience with deadlifting anyway. If you have strong fingers but weak forearms then this exercise might not help you at all. I guess the fingers are my weakest link in the deadlift.
My hands are large enough to hold the 60s, but any hex past that weight is too big for me to hold, so I compensate by putting rope through plates and tying it to the dumbbell.