This is the rear ride height adjustment strut.
A small change here will dramatically alter the character of the bike.
It's nearly impossible to comfortably adjust the ride height strut
without a special tool. I picked up the cheapest 19mm wrench
I could find, I think I paid two dollars. Then I just
ground off much of one tang until it was a good fit in
the recess where the adjusting nut is located.
I probably ground it thinner than necessary but it seems
plenty strong for the task.