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After much searching, I found documentation on SSA's Master Beneficiary Records format from after the Y2K update.

Date of Birth in some of these records is stored as days since January 1, 1800. There's no reference to 1875 anywhere. See section 2.3 Date formats:

https://omb.report/icr/200704-0960-012/doc/2576801

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But it was common to encode missing two-digits data as something in the high-90s, maybe specifying a reason in the least significant digit. (I was taught to do so at university and did so myself for a project.) So, if the start date is indeed 1880, the code for missing date could still have been 95.

I’m still kind of surprised they would store days since 1800, but it makes a lot more sense than 1875. At least with 1800 the last two digits are the same, so debugging it is somewhat sane.

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