When we build engines, we try our best to do it right and fail to check our own work. With a 2 stroke it's important to check compression and leakdown after the motor goes back together and before it's fitted in the frame. It's amazing how often there's a ding or scratch that we overlook that's just enough to cause a tiny air leak and that's often enough to melt it down.Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. Sadly I can't check because the engine is already in pieces because one big end bearing had seized. Doublechecking looks like a very good idea.
While the carbs are off, check and clean every jet and drilling to make sure they are clean and flow the same. Use a can of WD40 to spray through one drilling on one carb and repeat on the other carb and compare spray.