Antique Blue Plate with Fish

I have a blue plate that belonged to my mother's grandmother. She was born in the 1870's or 1880's. My mother was told the plate was from colonial times and my mother's family can trace their relatives back to the Mayflower. This plate measures 8.5" in diameter.There is a stamp in the middle of the back but I can't make out what it is. It has one crack that has been repaired. It is somewhat noticeable on the front and very noticeable on the back. It has a spring plate holder on it and I am wondering if I should try to remove it or just leave it in place. I would appreciate any thoughts on it's history and value. Thank you.