I've worked on a lot of pre-1900 houses (I even grew up in one) and the 2x4s from back then really were 2" x 4" instead of the modern 1.5" x 3.5". Two years ago I bought a house built in 1942 and I demolished one interior wall and re-used the studs from it to build some new walls. I kept building these walls 1/2" too tall even though I measured and re-measured the spaces I was putting them into very carefully. I eventually realized that these 1942 studs were not in fact 1.5" x 3.5" like I had been assuming, but were actually 1.75" x 3.75" (the extra 1/4" in width of the top and bottom plates of my walls is where the phantom extra 1/2" was coming from). So apparently there was a transitional period between the real 2" x 4" 2x4s and the 1.5" x 3.5" ones.

I discovered another weird transitional thing in this house. The old houses I worked on all had lath-and-plaster walls, with strips of rough wood lath covered with a thick rough plaster layer which was in turn covered with a thin smooth plaster layer. Modern houses of course use sheetrock, but my 1942 house covered the bare studs with 16" x 16" pre-formed interlocking blocks of 1" thick rough plaster, and then smooth plaster was laid over these blocks. I first encountered these when tearing down the ceiling in my kitchen, and these things were unbelievably fucking heavy. They basically weighed as much as solid stone of these dimensions, and I can't imagine what it must have been like to install them initially. It surely must have been a two-man job.

Edit: another fun experience I had was renovating an Atlanta house that had been built in 1843. When we tore down the original lath-and-plaster walls, we found embedded in every single wall and ceiling a single dead, flattened rat. That house must have stunk to high fucking hell when they first moved into it. I like to imagine that it had been built with slave labor and this was some well-deserved payback.