IIRC, the census prior to 1890’s were stored in vaults designed to protect against fire. But they were full and the Feds hadn’t built additional safe storage. 1890’s was stacked outside the vaults; the older stuff inside them was preserved. You’d think that all those Progressive administrations between 1890 and 1921 would have spent the pittance required for more quality storage. But they couldn’t be troubled to be competent. And they learned nothing in the next 50 years.