Could be a nickel blank (type 1 planchet, no upset rim). Could be a slug. But if it's the same dimensions and weight as a nickel, that seems a bit unlikely. Interesting!
Although there might appear to be a raised rim, the edge of the piece shows the correct 'cut and tear' marks that would be on a Type 1 Blank, and not on a Type 2 Planchet. Weight is close enough for a nickel blank or planchet, imo. photos are a bit dark to tell if there's a rim for sure...
Could be both. OP got a legitimate and desirable error, and somebody somewhere thinks they passed a slug for five cents.
Okay, now it looks like an upset rim to me -- but the edge texture still doesn't look right for that. But I haven't studied any of these types in-hand yet, so I'm watching here to learn.
The color and texture of the last 2 photos don't look like a nickel. Perhaps take some normal lighting photos next to some nickels.
It's a Type 2 Planchet from those last photos. What looked like 'cut/tear' marks in the first set of photos now looks like contact marks on the edge. You have nice example of a Type 2 Nickel Planchet.