Take a look at that stone for your ancestors who died in the 1840s. Does it look like it is made of the same material as other stones from that time, or does it have the look of stones from the early 1900s or even the look of stones from the late 20th century?
The stone may not be the original. A picture would obviously be a great to provide evidence of this. but if that is not possible make a note in your files that you do not think the stone is the original.
Stones made near the time of death can have errors. Ones carved a hundred later can as well.