Hard paper-did the corner snap test-you can hear it,
If you heard the sound of hard paper you did the test wrong. Seeing the hinge remnant makes me suspicious that you tried to do the test on that end. But as someone else noted, by 1883 American had moved away from hard paper all the way to a cotton paper with starch sizing. Strictly soft only.
From Siegel Sale 1137, Lot 312:
That item came into my collection from that sale.