I'm having trouble uploading a screenshot, but if you zoom into the D of dollar on the reverse of my 1880, you will see a raised lump at about 3 o'clock of the D.
As Kirkuleez mentions, 1879 and 1880 Proofs shared a reverse. Here's the lump being referred to: In addition, 1880 has a couple of obverse pickups; this die was apparently Proof-only. These are circled in red in my image - a little doubling on the crossbars of the 1, and a vertical die line at the neck/hair junction: Sometimes die markers are sufficient to nail down an attribution without question. This is such a case.
