If they missed this, this puts into question all the rest of the data IMO.
It would be in there, but smoothed over instead of the year-by-year entries that you are looking for.
For a lot of datasets like this that are before modern monitoring programs, you can often tell when something "big" happened in a certain time period or year from another source, but then you have to decide how to incorporate that with your long-range data that doesn't have year-by-year values. Sometimes it's best to just choose one method that covers all of the years and stick with it instead of modifying your model based on what you "think" is right from other sources.