They're the headbolt washers.
The only possibility that I can think of (if they did it on purpose) is that the person responsible was TRYING to shim the valve springs (done to increase spring tension, a bad idea for another thread) but did it on the wrong side of the assembly. I suppose they may have been trying to avoid buying the right size shims, but it's pretty evident that this would be WAY too thick. Did you happen to inspect you clearances prior to removing them?
Remove them and inspect all of the keepers for signs of damage from contact with the washers. Then measure and re-shim ALL of them... if they didn't have any clearance they've been eating up the cam lobes. Inspect the cam lobes for irregular surfaces too.