Thanks for the input everyone! It's certainly a heavily worn coin but I'm still leaning toward struck through grease/something being the cause of the missing lettering. It's the fact the issue is so localized to one area of the coin rather than more evenly distributed that makes me think it was a die event.
Also, woooh!!! My 100th post!!!
I love the community. Thank you all for being so helpful and supportive. Hopefully there will be plenty more posts to come.