Looking again, it might not be a grease issue. Not the 'I' how much smaller it is than next to the other devices. So that device could have been totally polished off. They have a real die enforcer back them He did a big number on a few dies during his time there:
Note the '1' on this one how it is thinned.
Another example of over polishing is this one:



The same worked may have over polished several of these dies during that time period, even the OP's coin.