I wish you luck getting that pin. My pin snapped within the first month of ownership. I am well below the stated weight limit and the seat bolts were tight. I requested a replacement pin from the distributor, YEAHBIKE. They played e-mail games with me for THREE MONTHS… always stating that the pin would come whenever I bugged them (nicely). I finally asked what was taking so long and they never replied back. Clowns!!! 
I resolved the problem by doing something I had intended to do all along. Inserted a thin rubber lining between the tube and the seat harness. Ensured that the seat was aligned (no pin to help with this now), and then tightened both clamp bolts. I then added a nut to the end of each clamp bolt and tightened these as well. This ensured that the threads running through the clamps didn’t strip due to possible over-tightening of the bolts. That seat goes nowhere now!