Willis Looking at the unadjusted plots leads me to suspect that there are 2 major changes in measurement methods/location.
This occur in january 1941 and June 1994 – The 1941 is well known (po to airport move) . I can find no connection for the 1994 shift
These plots show the 2 periods each giving a shift of 0.8C
The red line shows the effect of a suggested correction
This plot compares the GHCN corrected curve (green) to that suggested by me (red).
The difference between the 2 is approx 1C compared to the 2.5 you quote as the “cheat”.