This is the pin holding the seat back to the seat bottom bracket. The C clip comes off the pin groove very easily and is a problem to put back on – that is if you can find it!
My answer is to drill the pin for a small cotter pin and replace the washer with a thinner unit.
I also had to reverse the pin to move the cotter pin to the outside so it wouldn’t stick in the upholstery and being hidden by the hinge bracket, is a neater look. The sucker ain’t gonna come apart now:-)
Now on to other stuff.