Many years ago I went to FRA one afternoon on a B757.
En route I noted the OAT as -70 something which, on checking with the miniature Limitations chart, seemed to be one or two degrees outside the environmental envelope. I took advantage of the turnround to visit the wonderful (then) new multi million POUND met HQ. There I was informed, in no uncertain tones, that they had sent a sonde aloft from Paris at 0600 that morning and the temperature at that Flight Level
IS -66, so your aeroplane is wrong!
I thanked him very much for his help and returned home, still showing -73/4 (whatever).