As of 1999, NASA (red below) was doing a minimal amount of tampering with the US temperature record. They were only cooling the 1930’s by about 0.1C relative to present temperatures. Measured temperatures are green.
Compare that to their current version, which cools the 1930’s by nearly 0.7C relative to present temperatures.
According to NOAA documentation, they use the same TOBS correction as they were using in 1999. So the only reasonable explanation is that they are intentionally corrupting the temperature record.