200Tdi Pistons with exhaust valve clearance machining?

I just took apart a spare 200Tdi engine and there are round depressions on all four piston crowns corresponding to the exhaust valve heads. These look too uniform to be valve contact witness marks and there are concentric "machining" marks. When I offer the removed valve up to the depression it does not mate snugly suggesting that it is not the valve that caused it. I have seen intentional machining such as this on engines of other vehicle types but the shop photos of new pistons for the 200Tdi do not show this. Has anyone else noticed these marks?
Cheers.

What thickness head gasket did it have fitted? I ask as just wondering if its been modded in some way?

It had the zero-hole gasket which, from my other thread looks to be the thickest one. I suppose if one was modding then one might squeeze the lost volume down to a minimum or play around with injector proximity to get optimum fuel spray in the bowl. I don't know the engine's entire history as it is a spare one. A late EVC would leave the exhaust valve heads closer to the piston at TDC. Maybe the engine had a skipped timing belt and then repaired at some point previously but I haven't done the calcs to see how much interference and clouting would happen with one tooth of timing belt skipped.

No obvious machined pockets when I did my head gasket, seems strange that someone would go to the trouble of machining pockets in the pistons and then run the thickest gasket possible. I would have though a tooth out would have seen witness marks from the valves, I've seen "kissing" marks on the carbon from worn keyways. I guess someone could have been a bit keen when skimming the head, the manual lists the valve height relative to the head face, normally you would cut the seat to get it in tolerance not machine the piston although I have been guilty of taking a tiny amount off the valve head to get it back in spec after a skim.

Hi. Sorry for my excursion of wisdom. I have spoken to the previous owner of the engine and it seems I forgot that he told me he'd had a cam belt/piston/valve incident some time previously. So the depressions on the piston crowns are the exhaust valve collisions from his old set of exhaust valves hence why the new valves in the recon head did not match the damage. I have now measured the piston protrusion as requiring a one-hole gasket so I guess people might indeed be adopting a "safety factor" when selecting gaskets to cater for the uncorrected valve height. This increase in dead volume outside the piston bowl means less air crammed in the bowl and possibly lower bowl swirl affecting fuel spray mixing. I don't have a handle on how sensitive this engine is to that. X

200tdi and sensitive just arent in the same sentence