Ah ah ah! That's so funny to see that in the Radbmx site 30 years ago!
It was in Bicross Magazine, a frenchmag and I really wonder how my stopper ended here???
I won a free 6 months subscription with that invention
For the record, it worked really well on these levers and really blocked the wheel in a safe way and i never cut my finger.
Back in the days I loved doing front wheel stand, something I would never have done with "normal" stoppers!
For this stopper I remember using a 2mm diameter welding rod.