I'd say WW1 collar insignia? The square hole in the cent, hard to say, maybe some one needed a washer? I've been tempted myself when washers are 3 cents a piece!
Every time I have renovated a house, or built a new one, or a deck or porch or shed, I nailed a new penny to a stud, rafter or floor joist and write my name by it. Just a way to say "I was here" to whoever comes along years later