As PM98 said, it's struck thru grease/machinery oil. Nice example. (and before someone says it, it's not a Die Cap)
I agree with our fellow enthusiasts. Struck through grease. I have a die cap that is similar, but the face of the coin is wavy and the features/devices are significantly distorted rather than missing.
While two of our experts have already posted, I was going to say the same. The rim being undamaged is a good indicator.