Early 1800's had what is known as the year without a summer.
I think it was Tambora volcano  that exploded and cooled climate
It led to poor crops and hunger in many areas around the world.

I think there was another volcano around the same time that contributed to the dust in the air that led to cold temperatures and no doubt poor growing conditions for your tree as well as other plants.

That was quite an age.
Have counted rings on a large spruce tree and got around 100 that put it starting around 1900
There can be quite a difference in size of rings from different years
I have seen a trunk from a plantation of pine where rings got smaller and smaller until it was cut in a management cut to thin out and let best trees grow quicker. It was about 30 years old so competition can be one reason for small rings too.