Well, from my (admittedly limited) knowledge the problem on the 600 comes from some sort of vibration mode caused by the unique combination of the bore, stroke, engine speed, weight of the piston and orientation. And probably the casting of the piston originally plays a part. I say this because the 750, 900, 620 etc aren't affected.
The damage is clearly visible on the outer surface of the lower (as the piston is oriented to ground when fitted in the bike) side piston skirt. It happens as the aluminium piston slides home into the cylinder, the skirt hit the bottom edge of the steel cylinder liner.
The lower face is the one affected simply due to gravity.
The fact that the ONLY damage on the pistons is in that area seems to imply some sort of flaring of the piston skirt (although as I type this I have thought of another possibility). From this I was wondering if I could simply shave the damaged area (a tiny bit) to remove the material that is interfering.
This obviously only makes any sense as an idea if the skirt is flared in the first place. I was planning to jig it up and take a clock to the surface to check this anyway
Oh - and don't worry; when I replace the piston (as is the 99% probable outcome) I have some (even more ridiculous) plans for the old piston.
Thanks for both of your quick replies though!
PS: this post wasn't supposed to be stating fact, more like me just thinking out loud.