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They found 86-dos 0.1 and 0.34, holy fucking shit!

https://archive.org/details/86-dos-version-0.1-c-serial-11-original-disk
https://archive.org/details/86-dos-v-0.34-221-81-02-20

MS bought the code of this operating system to create MS-DOS.
Therefore this is the oldest version of dos ever and probably one of the first ever created.