Hi,
A strange one but probably not totally without precedent. It looks like contact between the pushrod cap rim and the rocker.
Try turning the engine until the valve is fully open and see if there is clearance between the remaining bit of the push rod cap and the rocker. If there is contact or even very close then I would be thinking that there is something not quite right with the angles when the valve is fully open. A bit far fetched but could the cam be opening the valve too much and so allowing some contact? Are the inlet and exhaust rockers the same - they have different part numbers? If not and have been swapped, could a slightly different geometry cause this problem?
Dave.