There seems to be some confusion about whether Klein suggested 10 reps or 12. Mahler writes 12, Brooks Kubik writes 10. I haven't seen the original challenge, so I can't say who's correct, but I thought I'd mention it.
I seriously trained for this about twelve years ago. I got to 12 reps with 32 kg dumbells, then hit a plateau. Then, other priorities got in the way. Maybe I'll give it another shot again some time, but right now, it's certainly not my main focus. I've got an almost two decades old beef with the one arm pull-up to settle first.