Yep, for what it's worth it looks like planchet delamination to me. Note how the coin has hairline cracks along the rim spreading out from either side of the area where the piece of metal is missing. Had the delamination been worse you could've had quite a bit more of the coin reverse gone. There are several members here who are incredibly knowledgeable about large cents -- if one of them chimes in we might get more information about why this happens. I think it has something to do with the metal mixture of the coin but I could be in error. As a side note you can occasionally see this type of error right through to pennies of the early 1950's. A very nice find, john!