That was me when I was starting out ...

I am suspicious that you are right about vertical edge damage too.

As the piston comes up, the tightness begins at the point where the exhaust ports are about half covered.

I will report back on the success or otherwise of the #11 blade in rectifying it. If all else fails, you're on the other side of town and experienced with fixing these suckers ...

I also have a TD .051 piston & cylinder which I could drop in as a replacement.