fatmike (hope you don't mind me calling you that),

I'm no met but will try to give you an educated guess. Although the meteorologists in the past did not have the sophisticated tools available today, they, and mariners as well, were able to record barometric pressure readings, and I believe that the barometric pressure readings are what determines tropcial depressions, storms and hurricanes as well as their strength. Thus, if accurate barometric readings are available for past hurricanes, mets could calculate their relative strength.
Perhaps someone with more knowledge than I can chime in here.