Oh noes, it looks like some kind of detonation and local overheating to nibble away at the piston like that, and unless you have cleaned up the pistons and head they look remarkably clean, almost like they had been shotblasted. Do you think it was running very lean? What did the plugs look like?
I would say that the nibbling away on the piston matches the squish area between piston and head. It reminds me of my R&D days trying to get a Norton Wankel rotary to run on diesel, where detonation in the squish zone used to nibble away at the aluminium side plates just like that. I know nothing much about Landies, but one possibility is that the gasket was too thin for flat-top pistons? Are there low compression engines with different pistons that use a different gasket?
trevor