I picked up 4 of them at an auction for $55. I didn't know how I was going to hang them off the back of my 2210, I just knew I needed some weight. I mounted them on a cross drag bar that fits between the 3 pt. lift arms. I slid the cross drag bar through the handle section of the weight, and added a couple of bolts in the bar to act as stops to keep them centered. I liked the setup so much that I bought a couple more from the dealer (at $41 each). I can still mount at least two more, and will probably do so in the near future.
On a side note, the weights also "store" the spacing rod in the hole at the top of the slot. The spacing bar is needed on 2210's so the lift arms will also raise the MMM.