I believe that’s the NOAA data source and they started measuring this in the late 50’s. The direct measurement data doesn’t go further back because the technology to measure and or concern wasn’t there.

That said, people have been able to extrapolate past carbon ppm through means other than direct measurement.

https://theconversation.com/scientists-understood-physics-of-climate-change-in-the-1800s-thanks-to-a-woman-named-eunice-foote-164687#:~:text=Humans were already increasing carbon,carbon dioxide into the air.