What caused this - rusty dies?

This is a beautiful coin, but what is up with Britannia and the crusty-looking surface? Could a rusty die have caused this? It doesn't look like PMD, and NGC evidently doesn't think so either.
Disclaimer: This is not my coin and I have no affiliation with the seller. I kinda like it, although, I would like it better with a clean surface/strike.
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Comments
That doesn't keep a coin from be graded a high mint state, since it popped directly out the mint dies with those bumps.
<< <i>OOOH, also just noticed the slab states it is a 1/2p, not a farthing as the title/text suggests. >>
The slab is incorrect; that is a farthing, and I've yet to see very many even close to fully struck.
Gary
<< <i>Pics referenced in OP,
That coin is a real floordropper.I'm really amazed!
Aidan.