The temperature records are not retroactively falsified. There have been modifications based on improved knowledge but the net effect is pretty minimal. Just look at the HADCrut records yourself and you can see this claim is an out and out lie.
It is absolutely hilarious that someone would post temperature records that they believe prove their point on warming/cooling and then accuse those who generated those records of lying, effectively destroying their own argument in the process.
If the 20s and 30s were so warm, it appears the sea ice extent in the summer didn't notice
The sources for this older information is the digitising of news and other sources from the time:
The sea ice edge positions in the North Atlantic, between 1850 and 1978, derived from various sources, including newspapers, ship observations, aircraft observations, diaries and more.
Sea ice concentration data from regular aerial surveys of ice in the eastern Arctic by the Arctic and Antarctic Research Institute, St. Petersburg, Russia, beginning in 1933.
Sea ice edge positions for Newfoundland and the Canadian Maritime Region from observations, for 1870 to 1962.
Detailed charts of ice in the waters around Alaska for 1954 to 1978, originally the property of a consulting firm (the Dehn collection).
Arctic-wide maps of ice cover from the Danish Meteorological Institute from 1901 to 1956.
Whaling ship logbook entries that noted ship position along with an indication of whether the ship was in the presence of ice