It's a super-wide rim for balloon tires, so the offset is more than it appears if you think you're looking at a typical narrow rim.

I don't think the thin steel rim plays by all the same rules as modern aluminum rims. Once I get close to tension I typically adjust spokes only 1/8 of a turn at a time. I can build wheels that are perfectly true and round (well, within +/- 0.5mm) and evenly-tensioned with normal modern parts. But this thing functions differently. Main diffs:
* rim is likely more flexible, both vertically and torsionally
* amount of offset + torsional flexibility means rim can twist
* likely varying thickness of steel contributes to imperfections that show themselves increasingly under tension
* bike was in a 1949 tornado, found 1/2 mile away from the destroyed barn, and at least one rim is original to that time. My grandpa thinks he replaced the other rim, but isn't sure which one. Probably the front is original, since that thing had tons of bends in it. The rear (one that went from fairly true to less-true as I added tension) is likely the replacement, but still has significant variation in manufacturing imperfections.

Also, the increased out-of-true doesn't seem like a taco state. Just a gradual increase in out-of-true as tension increased.