Yes the Kessler is very good quality. Just bought a seal kit for it. Unfortunately I will be leaving for three months and probably cant do the rebuild until I return. Should be a potent old girl when its resealed.
I agree with the copper spacer, its awkward to get the barrel and tube to realign as the spacer will compress. It took me three attempts to get it right, the slightest overtightening will cause the front sight to end up on an angle, which makes accuracy impossible. The valve is well made, the ball bearing exhaust works well.
With each original Kessler there was a small envelope with extra spacers. You are supposed to add spacers to make it possible to completely tighten the assembly and still have the sights aligned. I've posted the original Kessler owners instructions which talks to this.
I suspect that Ron Sauls still has some of these copper spacers.