For your viewing pleasure, an Intrinsic metallic inclusion (It's a wee little one, but the best I could afford):
I think your coin is a Struck Through Foreign Object. A Intrinsic Metallic Inclusion will show up as a strip across a coin due to being in the metal before it is rolled to the correct thickness. The Inclusion in your coin has a roundish shape like a piece of metal got between the die and the planchet.
This is what you believed you have this is what intrinsic Metallic inclusion looks like. This specimen pulled from an 1971s no s Jefferson nickel proof set they are also known to have the cent ddo.the nickel I sold about two years ago or 67 deep camo! At one time I had 2 1971 proof sets each had the no s nickel. I broke the 1 and sold the other as I didn't want to be the guy to bust up the last full set no s 71 proof set.! There's 3 1971 proof Lincoln ddo's.,
It may look rounded in the images, but it's flush with the surface of the coin. If I thought it had any value in excess of the cost, I'd send it in for grading and get an official verdict. Mainly, I'm just waiting for Fred to weigh in and tell me it's PMD or a JPG artifact.
It would have to be flush with surface to be a Struck Through. Maybe square is a better word to use. Point being it's a spot not a streak.
One last musing on this; if it is a strike through, whatever the metal is has similar ductility as the coin's bronze, as it's surface is flat and even with the rest of that area of the coin. It's shape is irregular/oblong, roughly 2.17 x 1.44 mm. Edit: I suspect that's the limit of what I can learn about it. I can't think of any non-destructive method for finding out the type of metal. I suspect it's tin or zinc, probably tin, given it's appearance (considering the coin's age and its exposure to the atmosphere). I don't think an Xray spectrometer or similar device will have fine enough resolution to be able to get a reading from just that one bit of metal (unless we're talking NSA-grade equipment). Ah, well, one of life's minor mysteries.
