I don't think we've determined whether or not this is on a thick planchet. It's not clear to me whether "They were struck in the following combinations on both thick and thin planchets" from uspatterns applies to all of the compositions. A normal 1870 $1 should be 2.8mm thick. If this coin weighs the same as a regular 1870 (26.73 g) with the same diameter, then in order for it to be copper it has to be thicker. Silver is 1.17 times denser than copper so I'd expect somewhere around 3.25 mm if 'm doing the math right.