Let's take a look at a plot of BACKTO_1400-CENSORED vs. BACKTO_1400-FIXED vs. BACKTO_1400:
The red data and red trend line is the CENSORED version, which shows a downtrend, particularly in the 20th century ("hide the decline"?).
The green data and green trend line is the FIXED version, which shows a slight uptrend, particularly in the 20th century.
The blue data and blue trend line is the "BACKTO_1400"-no other label- file, which also shows a bit higher uptrend over the whole period than the FIXED version, but in the 20th century shows a slight downtrend, less than the CENSORED version.
It is also clear that the 3 datasets vary markedly in either direction on a yearly basis throughout the entire 1400-1980 period!
UPDATE: Looking at only the 20th century data produces the same trends, only amplified by a factor of 2.4! Also note the near perfect inverse symmetry of the linear regression lines (the regression equation for the CENSORED data is f(x)=0x - 2.39 and the regression for the FIXED data is f(x)=0x + 2.41). I would wager that selecting a slightly different date range will produce the perfect inverse regressions - i.e. "The Flip" noted here, or maybe the date range is correct and its just a rounding error by my spreadsheet program). What are the chances that the "adjustments" between the CENSORED and FIXED datasets are anything other than flipping the data (although it's more complicated than that because eyeball inspection shows the data are not inverse, but by some additional mathematical manipulations one can flip a data set to make the inverse look completely different, yet in the end result in the exact same regression line). With data files labeled CENSORED and FIXED, what are the chances that flipping the data was an innocent mistake, rather than deliberate? Looks like a smoking gun to me.
Next up, regressions on the 1400-1899 data.