i had the same problem but spotted it the first time,your piston rings are catching the power valve,i would put money on it that at some time when it was replated (i dont know what the bit is called but i will try and explain it to you )the olive/sleeve/collar that the end of the power valve shaft locates into which is deep in the barrel came out, this allowes the power valve to move and the rings catch it .this happened to mine 1000km after a re-plate
so it can happen at any time,the result was a piston that looks just like yours and a knackered barrel.Andy now has the barrel if you want a picture but you will need to removed the power valve shaft and look down the hole to see if it is down there or missing,you can do it with the barrrels on,if you want to know more i will pm you my number.